Fix traceback with malformed requisites

This fixes #6521 by skipping any requisites that have a value of None.
Additionally, the state output for failed requisites has been cleaned up
a bit. See the following link for an example:

https://github.com/saltstack/salt/issues/6521#issuecomment-22146254
Merge pull request #6525 from terminalmage/issue6521

Fix traceback with malformed requisites