fix: a default empty map for service bus variable (#1180)

Author: MacMur85

infrastructure/tf-core/variables.tf

@@ -458,6 +458,7 @@ variable "sqlserver" {
 
 variable "service_bus" {
   description = "Configuration for Service Bus namespaces and their topics"
+  default = {} 
   type = map(object({
     namespace_name   = optional(string)
     capacity         = number