Handle nil enable field in google_compute_router_peer without crashing

[edwardmedia]

Fixes hashicorp/terraform-provider-google#9891

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 a crash on `enable` field of `google_compute_router_peer`

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@rileykarson, please review this PR or find an appropriate assignee.

[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, 3 insertions(+))
Terraform Beta: Diff ( 2 files changed, 4 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComposerEnvironment_update|TestAccComposerEnvironment_withSoftwareConfig|TestAccPrivatecaCaPool_privatecaCapoolEmptyBaseline|TestAccTags You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=202639

[modular-magician]

Tests failed during RECORDING mode: TestAccPrivatecaCaPool_privatecaCapoolEmptyBaseline|TestAccComposerEnvironment_withSoftwareConfig|TestAccTags|TestAccComposerEnvironment_update Please fix these to complete your PR

[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, 3 insertions(+))
Terraform Beta: Diff ( 1 file changed, 3 insertions(+))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccPrivatecaCaPool_privatecaCapoolEmptyBaseline You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=203183

[rileykarson]

I looked into this a little more and now think true is the correct one, sorry about the churn!

[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, 3 insertions(+))
Terraform Beta: Diff ( 2 files changed, 5 insertions(+), 1 deletion(-))