There is a running difference in SerializerFeature:

public static final int WRITE_MAP_NULL_FEATURES
            = WriteMapNullValue.getMask()
            | WriteNullBooleanAsFalse.getMask()
            | WriteNullListAsEmpty.getMask()
            | WriteNullNumberAsZero.getMask()
            | WriteNullStringAsEmpty.getMask()
            ;

The lastest version will take WriteMapNullValue, WriteNullBooleanAsFalse, WriteNullListAsEmpty, WriteNullNumberAsZero, WriteNullStringAsEmpty as WriteMapNullValue while old version take SIMPLE WriteMapNullValue.

This will result different output when we set WriteNullStringAsEmpty with no WriteMapNullValue setted.
Nulled fields will be skipped in old versions but they will be "null" in newer versions.

Does it make sense?