Assume scratch disk size when not returned by the API

[c2thorn]

Fixes hashicorp/terraform-provider-google#7341

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 an issue where `google_compute_instance_template` would throw an error for unspecified `disk_size_gb` values while upgrading the provider.

[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, 12 insertions(+), 3 deletions(-))
Terraform Beta: Diff ( 1 file changed, 12 insertions(+), 3 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=148442"

[c2thorn]

Ran all the instance template tests in the TPGB MM upstream build, all passing.

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceDnsManagedZone_basic|TestAccProviderMeta_setModuleName|TestAccBigQueryJob_bigqueryJobCopyTableReferenceExample|TestAccComputeDisk_deleteDetach|TestAccComputeInstanceTemplate_withScratchDisk|TestAccComputeInstance_diskEncryption|TestAccComputeInstance_kmsDiskEncryption|TestAccComputeInstance_bootDisk_source|TestAccComputeInstance_bootDisk_sourceUrl|TestAccComputePerInstanceConfig_update|TestAccComputeRegionDisk_deleteDetach|TestAccComputeRegionPerInstanceConfig_update|TestAccComputeSnapshot_encryptionCMEK You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=148447"