Improve handling when enabling/disabling server TLS policy in global target HTTPS proxies

[modular-magician]

This PR implements similar behavior as in GoogleCloudPlatform/magic-modules#11184 but for global target HTTPS proxies.

Prior to this PR, you could enable a server TLS policy on a global HTTPS proxy, but disabling it would not work as removing server_tls_policy removes it from the PATCH payload entirely, so Google simply leaves the server TLS policy as is. What we actually need to do is send serverTlsPolicy = null if it's not set, which is what this PR does.

We're also adding a note about having to add the create_before_destroy = true lifecycle on google_network_security_server_tls_policy as removing the resource would make Terraform destroy the resource first before creating/updating other resources, and that would result in a "resource already in use" error. This lifecycle rule means Terraform will create/update resources (in this case it would update google_compute_target_https_proxy first and remove the association with the google_network_security_server_tls_policy resource) before destroying the resource.

I've updated the tests so that it creates the resources without a server TLS policy, then it adds the server_tls_policy and related resources, and then it removes them again. This is to ensure that we can successfully enable & disable mTLS.

Release Note Template for Downstream PRs (will be copied)

compute: allowed disabling 'server_tls_policy' during update in 'google_compute_target_https_proxy' resources

Derived from GoogleCloudPlatform/magic-modules#11496

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