What is the correct way in proto3 to use HasField?

Originally came up in #1329. HasField with proto3 fails on simple / singular / non-message fields.

@nathanielmanistaatgoogle Mentioned that @haberman might be able to shine some light. Josh, what should we be doing here? The current approach isn't great:

    # NOTE: As of proto3, HasField() only works for message fields, not for
    #       singular (non-message) fields. First try to use HasField and
    #       if it fails (with a ValueError) we manually consult the fields.
    try:
        return message_pb.HasField(property_name)
    except ValueError:
        all_fields = set([field.name for field in message_pb._fields])
        return property_name in all_fields

Should we bother with checking "has field", e.g.

if _has_field(key_pb.partition_id, 'dataset_id'):
    ...

or should we instead just check Truth-iness

if key_pb.partition_id.dataset_id:
    ...
# OR MORE STRICT
if key_pb.partition_id.dataset_id != '':
    ...