st/label: Don't guard against NULL text

Commit 9168f605 marked the parameter as nullable, which is in
conflict with the precondition check. But given that NULL is
valid for the underlying ClutterText (and GtkLabel accepts it
as well), there's no strong argument for disallowing NULL, so
remove the precondition check.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2465>