Fixes for BigQuery Adapter Integration

This release fixes the way that values are stored in Bigtable when the Typed Bigtable modules are used to create mutations. Initially the values were stored as byte strings, but there was an issue with the BigQuery adapter parsing boolean values. Also, the first attempt at clearing out columns by setting the value to empty strings did not work as BigQuery does not parse empty strings as NULL for value types other than STRING.

This release only impacts use by the Typed modules, not the raw GRPC requests.