kernclass: don't deref. NULL on empty chooser input

Fixes #1619.

r? @frank-trampe
I don't know for sure whether it's better to bypass all of GGadgetSetTitle8 and KCD_UpdateGlyphFromName if the name is null. It seemed like the safest thing to do, certainly, and the most appropriate to the function names (since it seems bad form to set a title to null or to update a null-named glyph), but I want to make sure that you agree with the approach.