Return ordinals from MultiOrdinals.MultiDocs

MultiOrdinals.MultiDocs returned 'null' ordinals which caused
a NPE if the field was single valued and would allow a significantly
smaller in memory representation than single packed int ordinals.

Closes #3470
Added random sort test for dense and sparse fields.

This test triggers a MultiDoc / MultiOrds in-memory representation
even if the field is not multivalued

Relates to #3470