[bugfix] Fixing regression from apache#4500 (apache#4549)

(cherry picked from commit ef4e5ec)

superset/viz.py

@@ -288,7 +288,7 @@ def get_payload(self, query_obj=None):
 
         df = payload.get('df')
         if self.status != utils.QueryStatus.FAILED:
-            if df is None or df.empty:
+            if df is not None and df.empty:
                 payload['error'] = 'No data'
             else:
                 payload['data'] = self.get_data(df)
@@ -611,7 +611,7 @@ def query_obj(self):
         return None
 
     def get_df(self, query_obj=None):
-        return pd.DataFrame()
+        return None
 
     def get_data(self, df):
         markup_type = self.form_data.get('markup_type')

tests/core_tests.py

@@ -922,7 +922,7 @@ def test_slice_payload_no_data(self):
 
         data = self.get_json_resp(url)
         self.assertEqual(data['status'], utils.QueryStatus.SUCCESS)
-        assert 'No data' in data['error']
+        self.assertEqual(data['error'], 'No data')
 
     def test_slice_payload_invalid_query(self):
         self.login(username='admin')
@@ -937,6 +937,15 @@ def test_slice_payload_invalid_query(self):
         self.assertEqual(data['status'], utils.QueryStatus.FAILED)
         assert 'KeyError' in data['stacktrace']
 
+    def test_slice_payload_viz_markdown(self):
+        self.login(username='admin')
+        slc = self.get_slice('Title', db.session)
+
+        url = slc.get_explore_url(base_url='/superset/explore_json')
+        data = self.get_json_resp(url)
+        self.assertEqual(data['status'], None)
+        self.assertEqual(data['error'], None)
+
 
 if __name__ == '__main__':
     unittest.main()