bigquery: fix TestAccBigQueryDataset_collationUpdate

[BBBmau]

We've been getting failures due to the collation diff failing when going from a value to a "" value.

------- Stdout: -------
=== RUN   TestAccBigQueryDataset_collationUpdate
=== PAUSE TestAccBigQueryDataset_collationUpdate
=== CONT  TestAccBigQueryDataset_collationUpdate
    resource_bigquery_dataset_test.go:592: Step 3/4 error: Check failed: Check 1/2 error: Check 1/1 error: google_bigquery_dataset.test: Attribute 'default_collation' expected "", got "und:ci"
--- FAIL: TestAccBigQueryDataset_collationUpdate (17.44s)
FAIL

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 901c591:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	1 file changed, 2 insertions(+), 8 deletions(-)
google-beta provider	View Diff	1 file changed, 2 insertions(+), 8 deletions(-)
terraform-google-conversion	View Diff	2 files changed, 4 insertions(+), 16 deletions(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
152	142	10	0

Affected Service Packages

• bigquery

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