compute/backend_bucket: Fix default behavior with USE_ORIGIN_HEADERS

[modular-magician]

Since the send_empty_value field breaks the use of "USE_ORIGIN_HEADERS" cache_mode, adding a custom encoder to remove those fields when necessary.

Cherry-picked from @Alan-pad's rework of a commit by @d10i in #7062

Fixes #10622
Fixes #13939
Closes #7062

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).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (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 with TTLs being sent when `USE_ORIGIN_HEADERS` is set

Derived from GoogleCloudPlatform/magic-modules#7588

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.