Add defaults for expr and description to prevent spurious diffs

[slevenick]

Fixes: hashicorp/terraform-provider-google#9084

Probably.

Problem sort of explained here (hashicorp/terraform-provider-google#9084 (comment)) where the state value seems to be changing between null & "" which causes the diff on the rule. Setting default values for both description & versioned_expr seems to fix this as far as I can tell

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed extra diffs generated on `google_security_policy` `rules` when modifying a rule

[modular-magician]

Hi! I'm the modular magician. Your PR hasn't generated any diffs, but I'll let you know if a future commit does.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 2 insertions(+))
Terraform Beta: Diff ( 1 file changed, 2 insertions(+))

[modular-magician]

Tests analytics

Total tests: 1995
Passed tests 1753
Skipped tests: 241
Failed tests: 1

Action taken

Triggering VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccBigQueryDataTable_bigtable

[modular-magician]

Tests passed during RECORDING mode:
TestAccBigQueryDataTable_bigtable[view]

All tests passed
View the build log or the debug log for each test

[rileykarson]

Unusual! I can't think of a way that this would break existing users so it should be a safe change, although needing it at all is a surprise in the first place.