add nil check to storagetransfer_job transfer schedule

[c2thorn]

Fixes hashicorp/terraform-provider-google#12697
fixes hashicorp/terraform-provider-google#12703

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)

storagetransfer: fixed a crash in `google_storagetransfer_job` refreshes when `transfer_schedule` was empty

[modular-magician]

Hi there, I'm the Modular magician - !

Diff report

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2197
Passed tests 1953
Skipped tests: 240
Failed tests: 4

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccComputeInstance_soleTenantNodeAffinities|TestAccCGCSnippet_eventarcWorkflowsExample|TestAccSqlDatabaseInstance_mysqlMajorVersionUpgrade

[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[Debug log]
TestAccSqlDatabaseInstance_mysqlMajorVersionUpgrade[Debug log]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[Error message] [Debug log]
TestAccCGCSnippet_eventarcWorkflowsExample[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[c2thorn]

confirmed the basic test locally