Is Avatica's ResultSetResponse's Signature field always present?

[F21]

I have a bug report for the Go Avatica driver where someone executed an 
`UPSERT` statement and caused the driver to crash. See 
https://github.com/Boostport/avatica/issues/34

The driver crashed, because we tried to read 
`ResultSetResponse.Signature` and it was null as the statement was an 
upsert statement.

According to the protobuf documentation [0], signature is non-optional 
and should always be present. Does this guarantee extend to data 
modification statements like UPSERT?

Cheers,

Francis


[0] 
https://calcite.apache.org/avatica/docs/protobuf_reference.html#resultsetresponse