Fixed NPE when "tools" key was missing in package_*_index.json file

Author: Federico Fissore

arduino-core/src/cc/arduino/contributions/packages/ContributedPackage.java

@@ -61,6 +61,9 @@ public ContributedPlatform findPlatform(String architecture, String version) {
   }
 
   public ContributedTool findTool(String name, String version) {
+    if (getTools() == null) {
+      return null;
+    }
     for (ContributedTool tool : getTools()) {
       if (tool.getName().equals(name) && tool.getVersion().equals(version))
         return tool;