[Rules] Allow a rule set with zero rules.

[afs]

@simonstey in #659 (comment)_

a file containing only prefixes would be invalid in this case, which is different from e.g. Turtle's Grammar (https://www.w3.org/TR/rdf12-turtle/#sec-grammar-grammar)

[1] turtleDoc  ::= statement*
[2] statement  ::= directive | (triples '.')

it's, however, aligned with SPARQL:

[1] QueryUnit  ::=  Query
[2] Query  ::=  Prologue( SelectQuery | ConstructQuery | DescribeQuery | AskQuery ) ValuesClause