allow empty state tiddlers and missing state attributes act as missing state tiddlers

As described here: https://groups.google.com/d/msg/tiddlywikidev/ODJhIw6-UjQ/3ZKrlA4B810J

I think it would be a good small enhancement as the reveal widget can now be used in previously impossible ways.
Merge pull request #1131 from Skeeve/patch-7

allow empty state tiddlers and missing state attributes act as missing s...