fix: update mongodb_version input to be optional in the DA and set default value to __NULL__ (which will display as preferred in the UI) (#484)

Author: arya-girish-k

ibm_catalog.json

@@ -183,8 +183,13 @@
             },
             {
               "key": "mongodb_version",
-              "required": true,
+              "required": false,
+              "default_value": "__NULL__",
               "options": [
+                {
+                "displayname": "preferred",
+                "value": "__NULL__"
+                },
                 {
                   "displayname": "6.0",
                   "value": "6.0"