Revert last commit

Last commit doesn't give the expected results so it's better not to have
it.

Author: Tadaboody

src/jupyter_contrib_nbextensions/nbextensions/varInspector/var_list.py

@@ -32,7 +32,7 @@ def _getshapeof(x):
         try:
             return x.shape
         except AttributeError: #x does not have a shape
-            return ''
+            return None
 
 def var_dic_list():
     types_to_exclude = ['module', 'function', 'builtin_function_or_method',