Unable to store nil value in BigQuery go sdk

Any struct type that  uses pointer type in any field is flag as invalid:

"Invalid underlying type: XXX "

With large BigQuery schema (200**** columns),  and large table size, ability of ingesting and enriching data from file system (GS, local)  where many record will have partial details with nulls is critial since it reduces cost, BigQuery does not charge for NULL, but does for 0, and "empty" messages, 

Imported from Jira [BEAM-5728](https://issues.apache.org/jira/browse/BEAM-5728). Original Jira may contain additional context.
Reported by: Witas.