Fix 'argument to reversed() must be a sequence' (apache#4237)

When passing empty/null location data out of certain rows in the spatial
control, Superset raises an error when trying to reverse the tuple.

superset/viz.py

@@ -1852,7 +1852,10 @@ def process_spatial_data_obj(self, key, df):
         elif spatial.get('type') == 'delimited':
             df[key] = (df[spatial.get('lonlatCol')].str.split(spatial.get('delimiter')))
             if spatial.get('reverseCheckbox'):
-                df[key] = [list(reversed(item))for item in df[key]]
+                df[key] = [
+                    tuple(reversed(o)) if isinstance(o, (list, tuple)) else (0, 0)
+                    for o in df[key]
+                ]
             del df[spatial.get('lonlatCol')]
         elif spatial.get('type') == 'geohash':
             latlong = df[spatial.get('geohashCol')].map(geohash.decode)