Disable/hide context menu items when no collections setup

[bruny]

When no collections have been setup, several context menu items don't have any relevance and should be hidden:

• Edit Rom Collection
• Delete Rom Collection
• Delete Game

These will throw exceptions when opened:

22:49:58 T:123145311502336   ERROR: EXCEPTION Thrown (PythonToCppException) : -->Python callback/script returned the following error<--
                                             - NOTE: IGNORING THIS CAN LEAD TO MEMORY LEAKS!
                                            Error Type: <type 'exceptions.AttributeError'>
                                            Error Contents: 'NoneType' object has no attribute 'keys'
                                            Traceback (most recent call last):
                                              File "/Users/bruny/Library/Application Support/Kodi/addons/script.games.rom.collection.browser/resources/lib/dialogeditromcollection.py", line 89, in onInit
                                                for rcId in self.romCollections.keys():
                                            AttributeError: 'NoneType' object has no attribute 'keys'
                                            -->End of Python script error report<--