GenericData compare fails for Maps size = 0 or sizes not same

Comparison of GenericData in the java avro library is incorrectly implemented.

Two bugs exists:

1) Shortcut returning of 'different' is wrong, since it compares m1.size() != m1.size() instead of m1.size() != m2.size()

2) When one of both maps are empty, the iterator throws an exception. There is no 'isEmpty' check.

See PR: https://github.com/apache/avro/pull/2325