billingbudget: make threshold optional

[DrFaust92]

Closes hashicorp/terraform-provider-google#12019

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)

billingbudget: made `thresholdRules` optional in `google_billing_budget`

[modular-magician]

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[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 102 insertions(+), 46 deletions(-))
Terraform Beta: Diff ( 3 files changed, 102 insertions(+), 46 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2095
Passed tests 1866
Skipped tests: 226
Failed tests: 3

Action taken

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

TestAccPrivatecaCertificateAuthority_subordinateCaActivatedByFirstPartyIssuerOnCreation|TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample|TestAccBillingBudget_billingBudgetOptionalExample

[modular-magician]

Tests passed during RECORDING mode:
TestAccBillingBudget_billingBudgetOptionalExample[view]

Tests failed during RECORDING mode:
TestAccPrivatecaCertificateAuthority_subordinateCaActivatedByFirstPartyIssuerOnCreation[view]
TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample[view]

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

[c2thorn]

edited changelog note to be past-tense and mentioned the specific resource affected