fixes a crash on updating log_bucket

[edwardmedia]

Fixes hashicorp/terraform-provider-google#13038

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Release Note Template for Downstream PRs (will be copied)

storage: fixed a crash when `log_bucket` is updated with empty body on `google_storage_bucket`

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[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.

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
TF Validator: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))

[ghost]

There should be some additional (or edited) test to go along with this change, no?

[modular-magician]

Tests analytics

Total tests: 2254
Passed tests 2013
Skipped tests: 241
Failed tests: 0

All tests passed in REPLAYING mode
View the build log

[c2thorn]

standard enough nil check, LGTM