dont show the missing icon/text if no missingIcon tiddler
dont show the missing icon/text if no missingIcon tiddler

Most of the time this is noise so it will now not display in the case of a
missing tiddler.
A user can bring it back by creating a missingIcon tiddler.

FND/tiddlyspace,TiddlySpace/tiddlyspace,FND/tiddlyspace,FND/tiddlyspace,TiddlySpace/tiddlyspace,TiddlySpace/tiddlyspace