[BEAM-12482] Pass null for schemaUpdateOptions

[crcornwell]

When writing temp tables during BigQuery batch loads, schema update options are invalid since the write disposition is always hard coded to WRITE_TRUNCATE. Including schema update options like this causes the load jobs to fail with the error:

"Schema update options should only be specified with WRITE_APPEND disposition, or with WRITE_TRUNCATE disposition on a table partition."

We could also address this by overriding the schema update options value for temp tables in the processElement function, similar to how its currently overriding the create and write dispositions. Let me know if that's preferred, or if I've totally missed the mark here otherwise.

R: @lukecwik
R: @chamikaramj

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[lukecwik]

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[lukecwik]

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[chamikaramj]

I don't think SchemaUpdateOptions can be just dropped.

See the corresponding fix to the Python side here: #14113

[lukecwik]

See #15281 (comment)

[crcornwell]

Ohh I see, so even though this small change would allow the temporary tables to be created, the copy job would still fail due to the schemas mismatching,

Okay, this is a bit chunkier than I anticipated, but I'll try to find some time to work on it.

[kennknowles]

Ah, I just came across this. Any update?

[aaltay]

What is the next step on this PR?

@crcornwell - are you still planning to work on it? Should we close it?

[crcornwell]

Yeah I'll go ahead and close it.