Implement serialization of KeyValueEntity without using MetaClass

[belyaev-andrey]

Environment

• Platform version: 7.x
• Addon version: 7.2.0

Description of the bug or enhancement

When a published service returns a KeyValue entity, its serialization to JSON fails because the metadata is null.
We do not add metadata when returning entities from datastore, but add it in the UI when adding KVEntity to the container:

com.haulmont.cuba.gui.model.impl.KeyValueCollectionContainerImpl#updateEntityMetadata

During serialization we invoke

com.haulmont.cuba.core.global.MetadataTools#isEmbeddable(com.haulmont.chile.core.model.MetaClass) 

And got NPE.

See https://www.cuba-platform.com/discuss/t/exception-in-rest-controller/12424

• Minimal reproducible example
  The code attached. RestApiThouthgs.zip

Run the application add a Customer and then try to invoke a published service using http://localhost:8080/app/rest/v2/services/restapithouthgs_RestService/getBuyer URL

• Expected behavior
  Entity should be returned

• Actual behavior

22:06:21.009 ERROR c.h.a.r.a.c.RestControllerExceptionHandler- Exception in REST controller
java.lang.IllegalStateException: metaClass is null
	at com.haulmont.cuba.core.entity.KeyValueEntity.getMetaClass(KeyValueEntity.java:71) ~[cuba-global-7.2.4.jar:7.2.4]
	at com.haulmont.cuba.core.app.serialization.EntitySerialization$EntitySerializer.serializeEntity(EntitySerialization.java:217) ~[cuba-global-7.2.4.jar:7.2.4]
	at com.haulmont.cuba.core.app.serialization.EntitySerialization$EntitySerializer.serialize(EntitySerialization.java:212) ~[cuba-global-7.2.4.jar:7.2.4]
	at com.haulmont.cuba.core.app.serialization.EntitySerialization$EntitySerializer.serialize(EntitySerialization.java:189) ~[cuba-global-7.2.4.jar:7.2.4]
	at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:97) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:61) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.write(MapTypeAdapterFactory.java:208) ~[gson-2.8.6.jar:na]
	at com.google.gson.internal.bind.MapTypeAdapterFactory$Adapter.write(MapTypeAdapterFactory.java:145) ~[gson-2.8.6.jar:na]
	at com.google.gson.Gson.toJson(Gson.java:704) ~[gson-2.8.6.jar:na]
	at com.google.gson.Gson.toJson(Gson.java:683) ~[gson-2.8.6.jar:na]
	at com.google.gson.Gson.toJson(Gson.java:638) ~[gson-2.8.6.jar:na]
	at com.google.gson.Gson.toJson(Gson.java:618) ~[gson-2.8.6.jar:na]
	at com.haulmont.cuba.core.app.serialization.EntitySerialization.objectToJson(EntitySerialization.java:115) ~[cuba-global-7.2.4.jar:7.2.4]
	at com.haulmont.addon.restapi.api.common.RestParseUtils.serialize(RestParseUtils.java:126) ~[restapi-rest-api-7.2.0.jar:na]
	at com.haulmont.addon.restapi.api.service.ServicesControllerManager._invokeServiceMethod(ServicesControllerManager.java:189) ~[restapi-rest-api-7.2.0.jar:na]
	at com.haulmont.addon.restapi.api.service.ServicesControllerManager.invokeServiceMethodGet(ServicesControllerManager.java:83) ~[restapi-rest-api-7.2.0.jar:na]
	at com.haulmont.addon.restapi.api.controllers.ServicesController.invokeServiceMethodGet(ServicesController.java:62) ~[restapi-rest-api-7.2.0.jar:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_172]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_172]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_172]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_172]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:888) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) [spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) [spring-webmvc-5.2.3.RELEASE.jar:5.2.3.RELEASE]

[knstvk]

KeyValueEntity can be obtained from different sources or created on-the-fly. So we cannot rely on presence of MetaClass in it. The only reliable solution is to serialize to JSON without using MetaClass at all.