Fix U4-2320 - Error on saving User Permissions

Nodes can be null but NodeIds property always returns a collection and this will return Ids from Nodes if _nodeIds are null and Nodes is not.
Updated RenderMvcController to have the view engines test for file existance, rather than assuming views will always be razor. This allows alternative MVC view engines to be used.
Merge pull request #7 from pynej/6.1.2

Fixed Ctl+S on stylesheets editor.
Merge pull request #6 from dpen2000/UserPermisionsFixIn6.1.2

Fix U4-2320 - Error on saving User Permissions (for 6.1.2 release)
Merge pull request #8 from mattbrailsford/6.1.2

Updated RenderMvcController to have the view engines test for file existance