64bit fields no longer support `None` values in v1.19.6

[larkee]

The Cloud Spanner emulator tests began failing yesterday with the following error:

tests/system/test_session_api.py:2150: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
google/cloud/spanner_v1/database.py:1127: in generate_query_batches
    partitions = self._get_snapshot().partition_query(
google/cloud/spanner_v1/snapshot.py:525: in partition_query
    partition_options = PartitionOptions(
.nox/system-3-8/lib/python3.8/site-packages/proto/message.py:516: in __init__
    pb_value = marshal.to_proto(pb_type, value)
.nox/system-3-8/lib/python3.8/site-packages/proto/marshal/marshal.py:211: in to_proto
    pb_value = rule.to_proto(value)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <proto.marshal.rules.stringy_numbers.Int64Rule object at 0x7fd4dcc59940>
value = None

    def to_proto(self, value):
>       return self._python_type(value)
E       TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

.nox/system-3-8/lib/python3.8/site-packages/proto/marshal/rules/stringy_numbers.py:34: TypeError

I have confirmed that this isn't an issue when using v1.19.5.

[tseaver]

Regression introduced in #267.