support case where env is None

when I use this module I get an exception 

    State: - rabbitmq_plugins
    Name:      rabbitmq_management
    Function:  enabled
        Result:    False
        Comment:   An exception occurred in this state: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1280, in call
    ret = self.states[cdata['full']](*cdata['args'])
  File "/var/cache/salt/minion/extmods/states/rabbitmq_plugins.py", line 65, in enabled
    plugins = __salt__['rabbitmq_plugins.list'](env=env, runas=runas)
  File "/var/cache/salt/minion/extmods/modules/rabbitmq_plugins.py", line 52, in list
    env=_convert_env(env))
  File "/var/cache/salt/minion/extmods/modules/rabbitmq_plugins.py", line 27, in _convert_env
    for var in env.split():
AttributeError: 'NoneType' object has no attribute 'split'