feat: allow queryJob.result() to be called on a dryRun (#1015)

* feat: allow queryJob.result() to be called on a dryRun

* update to return EmptyRowIterator

google/cloud/bigquery/job/query.py

@@ -1318,6 +1318,8 @@ def result(
                 If Non-``None`` and non-default ``job_retry`` is
                 provided and the job is not retryable.
         """
+        if self.dry_run:
+            return _EmptyRowIterator()
         try:
             retry_do_query = getattr(self, "_retry_do_query", None)
             if retry_do_query is not None:

tests/unit/job/test_query.py

@@ -26,6 +26,7 @@
 
 from google.cloud.bigquery.client import _LIST_ROWS_FROM_QUERY_RESULTS_FIELDS
 import google.cloud.bigquery.query
+from google.cloud.bigquery.table import _EmptyRowIterator
 
 from ..helpers import make_connection
 
@@ -989,6 +990,19 @@ def test_result(self):
             [query_results_call, query_results_call, reload_call, query_page_call]
         )
 
+    def test_result_dry_run(self):
+        job_resource = self._make_resource(started=True, location="EU")
+        job_resource["configuration"]["dryRun"] = True
+        conn = make_connection()
+        client = _make_client(self.PROJECT, connection=conn)
+        job = self._get_target_class().from_api_repr(job_resource, client)
+
+        result = job.result()
+
+        calls = conn.api_request.mock_calls
+        self.assertIsInstance(result, _EmptyRowIterator)
+        self.assertEqual(calls, [])
+
     def test_result_with_done_job_calls_get_query_results(self):
         query_resource_done = {
             "jobComplete": True,