fix: Don't define backup_configuration if default DB is configured as a replica

[OscarVanL]

I made a mistake in my last PR #556 which allow backups to be created if the default DB is configured as a replica (via master_instance_name), when this is not a valid configuration.

CloudSQL does not allow you to create backups of replicas, so this block should be blanked if the intance is configured as a replica.

[imrannayer]

/gcbrun

[imrannayer]

@OscarVanL integration test failed with following error:

TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: Error: Reference to undeclared input variable
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: 
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185:   on ../../../modules/mysql/main.tf line 71, in resource "google_sql_database_instance" "default":
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185:   71:       for_each = var.master_instance_name != null ? [] : [var.backup_configuration]
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: 
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: An input variable with the name "master_instance_name" has not been declared.
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: This variable can be declared with a variable "master_instance_name" {}
TestMySqlHaModule 2023-12-19T14:08:05Z command.go:185: block.
TestMySqlHaModule 2023-12-19T14:08:05Z retry.go:99: Returning due to fatal error: FatalError{Underlying: error while running command: exit status 1; 
Error: Reference to undeclared input variable

  on ../../../modules/mysql/main.tf line 71, in resource "google_sql_database_instance" "default":
  71:       for_each = var.master_instance_name != null ? [] : [var.backup_configuration]

An input variable with the name "master_instance_name" has not been declared.
This variable can be declared with a variable "master_instance_name" {}
block.}
    validate.go:11: 
        	Error Trace:	/builder/home/go/pkg/mod/github.com/gruntwork-io/terratest@v0.46.7/modules/terraform/validate.go:11
        	            				/builder/home/go/pkg/mod/github.com/!google!cloud!platform/cloud-foundation-toolkit/infra/blueprint-test@v0.10.2/pkg/tft/terraform.go:411
        	            				/builder/home/go/pkg/mod/github.com/!google!cloud!platform/cloud-foundation-toolkit/infra/blueprint-test@v0.10.2/pkg/tft/terraform.go:442
        	            				/builder/home/go/pkg/mod/github.com/!google!cloud!platform/cloud-foundation-toolkit/infra/blueprint-test@v0.10.2/pkg/tft/terraform.go:469
        	            				/builder/home/go/pkg/mod/github.com/!google!cloud!platform/cloud-foundation-toolkit/infra/blueprint-test@v0.10.2/pkg/utils/stages.go:31
        	            				/builder/home/go/pkg/mod/github.com/!google!cloud!platform/cloud-foundation-toolkit/infra/blueprint-test@v0.10.2/pkg/tft/terraform.go:469
        	Error:      	Received unexpected error:
        	            	FatalError{Underlying: error while running command: exit status 1; 
        	            	Error: Reference to undeclared input variable
        	            	
        	            	  on ../../../modules/mysql/main.tf line 71, in resource "google_sql_database_instance" "default":
        	            	  71:       for_each = var.master_instance_name != null ? [] : [var.backup_configuration]
        	            	
        	            	An input variable with the name "master_instance_name" has not been declared.
        	            	This variable can be declared with a variable "master_instance_name" {}
        	            	block.}
        	Test:       	TestMySqlHaModule
--- FAIL: TestMySqlHaModule (9.55s)
FAIL

[OscarVanL]

@imrannayer What version of the module is that testing against? That variable was only added in my last commit: 6cda644, so if it is pinned on the latest release (v18.1.0) that wouldn't include the variable. That variable will exist if it runs against HEAD of this repo.

[imrannayer]

/gcbrun