ValueRule.to_python fails with empty Value field

[dizcology]

An example:

from google.protobuf import struct_pb2

import proto

class Model(proto.Message):
    metadata: struct_pb2.Value = proto.Field(
        proto.MESSAGE,
        number=6,
        message=struct_pb2.Value,
    )

empty_value = struct_pb2.Value()
m = Model(metadata=empty_value)

print('metadata' in m)  # not absent
print(m._pb.metadata.WhichOneof('kind'))  # None Value kind
print(m._pb.metadata == empty_value)  # The empty Value can be accessed through `_pb`
m.metadata  # But this fails with: ValueError: Unexpected kind: None

The error is raised at

proto-plus-python/proto/marshal/rules/struct.py

Line 62 in 9270fe5

raise ValueError("Unexpected kind: %s" % kind) # pragma: NO COVER

In the situation above, absent is False, while kind is None.

My expectation is that since protobuf allows the construction of empty Values, proto-plus should too. However, it is unclear to me what should be the return value of to_python in this case.