Fix Uncaught TypeError: Cannot call method 'substr' of undefined

Fix Uncaught TypeError: Cannot call method 'substr' of undefined
* When changing the name of a category and saving it, this error is thrown from $.ucFirst. Although the script does check for it to be a string, what happens between that and ucFirst is broken. It should always set 'category' of splitresult[0], not just if there is a [1] (=sortkey). Otherwise, if there is no sortkey, 'category' is undefined.
* Also updated outdated comment saying that variable sortkey contains wikitext between "[[Category:Foo" and "]]" (which implies it includes the pipe, which is no longer true)

Follows-up r93351.