Revert "fixed missing tab — tids may be empty but existing"

Reverts TiddlyWiki/tiddlywiki#149

After further examination, this change is not correct, as it doesn't allow for section references in transclusions (e.g., "<<tiddler TiddlerName##section>>"), which would then be reported as missing.   Also, if a tiddler exists, but is empty of content, then the tiddler.text property should NOT be null, but rather a blank text string  (e.g., "").  The only way for a tiddler to have a null text property is if it was improperly created or updated by a plugin that passes a null value, rather than a blank text string when using store.saveTiddler(...).