Fix pydotprint_variables bug with pydot 1.0.28.

theano/printing.py

@@ -949,7 +949,7 @@ def pydotprint_variables(vars,
                     g.add_node(pd.Node(varastr, color='green'))
             else:
                 varastr = my_list[nd]
-            label = ''
+            label = None
             if len(app.inputs) > 1:
                 label = str(i)
             g.add_edge(pd.Edge(varastr, astr, label=label))
@@ -974,7 +974,7 @@ def pydotprint_variables(vars,
                     g.add_node(pd.Node(varastr, color=color))
             else:
                 varastr = my_list[nd]
-            label = ''
+            label = None
             if len(app.outputs) > 1:
                 label = str(i)
             g.add_edge(pd.Edge(astr, varastr, label=label))