Checks if groups is none before extending it

This prevents getting an AttributeError when trying to create a new user when groups is None:

----------
   State: - user
   Name:      git
   Function:  present
       Result:    False
       Comment:   An exception occured in this state: Traceback (most recent call last):
 File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1205, in call
   ret = self.states[cdata['full']](*cdata['args'])
 File "/usr/lib/python2.7/dist-packages/salt/states/user.py", line 305, in present
   groups.extend(present_optgroups)
AttributeError: 'NoneType' object has no attribute 'extend'