Make field mapping function parameters nullable. (Azure#15273)

Author: bleroy

specification/search/data-plane/Azure.Search/preview/2021-04-30-Preview/searchservice.json

@@ -6345,6 +6345,7 @@
         },
         "parameters": {
           "type": "object",
+          "x-nullable": true,
           "additionalProperties": {
             "type": "object"
           },