[AMS] Make primaryVerificationKey nullable (#20489)

Author: giakas

specification/mediaservices/resource-manager/Microsoft.Media/stable/2022-08-01/ContentKeyPolicies.json

@@ -498,7 +498,8 @@
         },
         "primaryVerificationKey": {
           "$ref": "#/definitions/ContentKeyPolicyRestrictionTokenKey",
-          "description": "The primary verification key."
+          "description": "The primary verification key.",
+          "x-nullable": true
         },
         "alternateVerificationKeys": {
           "type": "array",