Add support for column specific fillna to viz (#3066)

Fix #3029

tests/core_tests.py

@@ -767,5 +767,16 @@ class CoreTests(SupersetTestCase):
         assert 'language' in resp
         self.logout();
 
+    def test_viz_get_fillna_for_columns(self):
+        slc = self.get_slice("Girls", db.session)
+        q = slc.viz.query_obj()
+        results = slc.viz.datasource.query(q)
+        fillna_columns = slc.viz.get_fillna_for_columns(results.df.columns)
+        self.assertDictEqual(
+            fillna_columns,
+            {'name': ' NULL', 'sum__num': 0}
+        )
+
+
 if __name__ == '__main__':
     unittest.main()