fix: Correct DB instance parameter group used when users do not create one nor provide the name of existing, will use default (#321)

Author: bryantbiggs

README.md

@@ -307,7 +307,7 @@ No modules.
 | <a name="input_db_cluster_parameter_group_use_name_prefix"></a> [db\_cluster\_parameter\_group\_use\_name\_prefix](#input\_db\_cluster\_parameter\_group\_use\_name\_prefix) | Determines whether the DB cluster parameter group name is used as a prefix | `bool` | `true` | no |
 | <a name="input_db_parameter_group_description"></a> [db\_parameter\_group\_description](#input\_db\_parameter\_group\_description) | The description of the DB parameter group. Defaults to "Managed by Terraform" | `string` | `null` | no |
 | <a name="input_db_parameter_group_family"></a> [db\_parameter\_group\_family](#input\_db\_parameter\_group\_family) | The family of the DB parameter group | `string` | `""` | no |
-| <a name="input_db_parameter_group_name"></a> [db\_parameter\_group\_name](#input\_db\_parameter\_group\_name) | The name of the DB parameter group | `string` | `""` | no |
+| <a name="input_db_parameter_group_name"></a> [db\_parameter\_group\_name](#input\_db\_parameter\_group\_name) | The name of the DB parameter group | `string` | `null` | no |
 | <a name="input_db_parameter_group_parameters"></a> [db\_parameter\_group\_parameters](#input\_db\_parameter\_group\_parameters) | A list of DB parameters to apply. Note that parameters may differ from a family to an other | `list(map(string))` | `[]` | no |
 | <a name="input_db_parameter_group_use_name_prefix"></a> [db\_parameter\_group\_use\_name\_prefix](#input\_db\_parameter\_group\_use\_name\_prefix) | Determines whether the DB parameter group name is used as a prefix | `bool` | `true` | no |
 | <a name="input_db_subnet_group_name"></a> [db\_subnet\_group\_name](#input\_db\_subnet\_group\_name) | The name of the subnet group name (existing or created) | `string` | `""` | no |

main.tf

@@ -176,7 +176,7 @@ resource "aws_rds_cluster_instance" "this" {
   ca_cert_identifier                    = var.ca_cert_identifier
   cluster_identifier                    = aws_rds_cluster.this[0].id
   copy_tags_to_snapshot                 = try(each.value.copy_tags_to_snapshot, var.copy_tags_to_snapshot)
-  db_parameter_group_name               = var.create_db_parameter_group ? aws_db_parameter_group.this[0].id : local.db_parameter_group_name
+  db_parameter_group_name               = var.create_db_parameter_group ? aws_db_parameter_group.this[0].id : var.db_parameter_group_name
   db_subnet_group_name                  = local.db_subnet_group_name
   engine                                = var.engine
   engine_version                        = var.engine_version

variables.tf

@@ -341,7 +341,7 @@ variable "ca_cert_identifier" {
 variable "db_parameter_group_name" {
   description = "The name of the DB parameter group"
   type        = string
-  default     = ""
+  default     = null
 }
 
 variable "instances_use_identifier_prefix" {