tgc: update bigquery dataset fixtures for default_collation

[c2thorn]

Update BigQuery dataset TGC test fixtures as a followup to #18585.

Because default_collation is no longer computed, TGC now includes "defaultCollation": "" in the asset output for unconfigured datasets.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit a724cee:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
terraform-google-conversion	View Diff	8 files changed, 8 insertions(+)

[c2thorn]

@modular-magician reassign-reviewer

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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