This means (use-package foopkg :mode (".foo")) will add (".foo"
. foopkg) into auto-mode-alist instead of the broken (".foo" . nil),
this is more consistent with the behaviour of (use-package foopkg
:mode (".foo" ".bar")).
This is not a pure refactoring, it also fixes a bug where
:bind ([keysym] . "string") would actually bind keysym to nil (i.e.,
unbind it). It now binds to "string" as expected.