Fix Null Pointer Exception in BigQueryIO

[tarun-google]

Fixes #33074

Part-1: Check customGcsTempLocation value provider wrapping a null location. This is happening in Managed FileLoads path. While the next solution is to fix from client. This is part-1 fixes the BigQueryIO to be more robust in checking the null wrapped ValueProvider. We can add a stricter check that validates the value of customGcsTempLocation before hand. But we need to evaluate more paths and fix the client first.

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R: @m-trieu for label java.

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[tarun-google]

R: @chamikaramj @ahmedabu98

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[ahmedabu98]

LGTM

[Abacn]

valueProvider.get() check at pipeline submission time may not work: GoogleCloudPlatform/DataflowTemplates#2892 (comment)