provide blank links where space doesn't exist
provide blank links where space doesn't exist

otherwise tests fail (for example test_friendly_uri.py) where tiddlers
do not have modifier or creator attributes set.

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