fix: update rabbitmq_version input to be optional in the DA and set default value to __NULL__ (which will display as preferred in the UI) (#314)

Author: arya-girish-k

ibm_catalog.json

@@ -189,8 +189,13 @@
               },
               {
                 "key": "rabbitmq_version",
-                "required": true,
+                "required": false,
+                "default_value": "__NULL__",
                 "options": [
+                  {
+                  "displayname": "preferred",
+                  "value": "__NULL__"
+                  },
                   {
                     "displayname": "6.2",
                     "value": "6.2"