[Synapse] - Monitoring API should allow sparkJobDefinition and pipeline to be null (#11165)

Co-authored-by: Dongwei Wang <[email protected]>

Author: idear1203

specification/synapse/data-plane/Microsoft.Synapse/preview/2019-11-01-preview/monitoring.json

@@ -162,10 +162,12 @@
           }
         },
         "sparkJobDefinition": {
-          "type": "string"
+          "type": "string",
+          "x-nullable": true
         },
         "pipeline": {
           "uniqueItems": false,
+          "x-nullable": true,
           "type": "array",
           "items": {
             "$ref": "#/definitions/SparkJob"