Fix unnecessary merge comparison for required fields
Fix unnecessary merge comparison for required fields

In the generated code, functions merge_msg_* contained an unnecessary
comparison with undefined for required (scalar) fields. They were
treated the same way as optional (scalar) fields in this respect, but
that was not necessary (and dialyzer in Erlang 19 finds it due to the
removal of the implicit "| undefined" for record fields)

mmmries/gpb