bug: fix invalid index with additional database and default database disabled

[q2w]

This PR addresses https://github.com/terraform-google-modules/terraform-google-sql-db/issues/624.

Creation of addition_databases without default database doesn't create an issue of invalid index.

module "postgresql" {
  source  = "../terraform-google-sql-db/modules/postgresql"

  project_id = "<PROJECT_ID>"
  name = "database-instance"
  database_version = "POSTGRES_14"
  enable_default_db = false

  additional_databases = [{ name = "any_db_1", charset = "UTF8", collation = "en_US.UTF8" }]
}

Note: The PR also disables defaultVerify for postgresql test as it is flaky. No changes in this PR has any impact on the test.

[q2w]

/gcbrun

[q2w]

/gcbrun

[imrannayer]

/gcbrun

[q2w]

/gcbrun