Fix crash caused by nil value in BigQuery Table

[slevenick]

Fixes: hashicorp/terraform-provider-google#17708

I don't want to add a test because it's impossible to provision a table with an empty string as the column. This now just triggers the correct 400 from the API rather than crashing

Release Note Template for Downstream PRs (will be copied)

bigquery: fixed a crash when `google_bigquery_table` had a `primary_key.columns` entry set to `""`

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 5 insertions(+))
google-beta provider: Diff ( 1 file changed, 5 insertions(+))

[modular-magician]

Tests analytics

Total tests: 113
Passed tests: 102
Skipped tests: 11
Affected tests: 0

Click here to see the affected service packages

• bigquery

$\textcolor{green}{\textsf{All tests passed!}}$
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[zli82016]

LGTM