Fix logic error if there are no tables

Author: mwcraig

astrowidgets/ginga.py

@@ -564,6 +564,9 @@ def get_markers(self, x_colname='x', y_colname='y',
                     del table[skycoord_colname]
                 tables.append(table)
 
+            if len(tables) == 0:
+                return None
+
             stacked = vstack(tables, join_type='exact')
 
             if coordinates: