protect PluginLoader._extra_dirs from appending None

PluginLoader.add_directory() can receive None from, for example, 
Inventory.add_directory(self.basedir()) if host_list is a custom list.
None has no reasonable interpretation other than ignore it.

lib/ansible/utils/plugins.py

@@ -70,6 +70,7 @@ class PluginLoader(object):
 
     def add_directory(self, directory):
         """Adds an additional directory to the search path"""
+        if directory is not None:
             self._extra_dirs.append(directory)
 
     def print_paths(self):