fix error in BigQueryUtils when retrieving non existent avro field

[tilgalas]

fixes #30730
fixes #30729

An integration test called BigQueryIOStorageReadIT.testBigQueryStorageReadProjectionPushdown is failing - this is because the pushdown optimizer projects the record retrieved from the bigquery by removing unused string_field. Later the toBeamRow conversion method iterates through the beam schema's fields and tries to extract a value from the avro record for every such field, including the one that has been removed by the optimizer - avro version 1.8 didn't have a problem with attempting to get a value from non-existent field - it simply returned null, but things have changed in version 1.11, and now this operation throws an exception. The fix is to check whether the field exists in the record before attempting to fetch its value. Also - added a unit test specifically for this situation

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[tilgalas]

Run PostCommit_Java_Dataflow

[Abacn]

V2 PostCommit passed; V1 PostCommit failed quota issue on unrelated tests, merging for now