batches: applying a campaign against a 3.25.x Sourcegraph instance fails

Created by: LawnGnome

Previewing is fine, so this is something specific to the invocation of the apply mutation:

   {
     "locations": [
       {
         "column": 27,
         "line": 2
       }
     ],
     "message": "Variable \"batchSpec\" has invalid value null.\nExpected type \"ID!\", found null."
   }
   {
     "locations": [
       {
         "column": 2,
         "line": 3
       }
     ],
     "message": "Cannot query field \"applyBatchChange\" on type \"Mutation\"."
   }
   {
     "locations": [
       {
         "column": 3,
         "line": 4
       }
     ],
     "message": "Fragment \"batchChangeFields\" cannot be spread here as objects of type %!q(\u003cnil\u003e) can never be of type %!q(\u003cnil\u003e)."
   }
   {
     "locations": [
       {
         "column": 31,
         "line": 8
       }
     ],
     "message": "Unknown type \"BatchChange\"."
   }
   {
     "locations": [
       {
         "column": 9,
         "line": 11
       }
     ],
     "message": "Fragment \"namespaceFields\" cannot be spread here as objects of type %!q(\u003cnil\u003e) can never be of type \"Namespace\"."
   }