Add nil checks and treat "null" as "[]" in bigQueryTableSchemaDiffSup…

[modular-magician]

…press

Fixes hashicorp/terraform-provider-google#7302

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Release Note Template for Downstream PRs (will be copied)

bigquery: fixed an issue where `google_bigquery_table` would crash while reading an empty schema

Derived from GoogleCloudPlatform/magic-modules#4017