keyboard: Don't check for key.action using strict equality operator

keyboard: Don't check for key.action using strict equality operator

A lot of keys have no action set. In that case key.action is `undefined`,
but the strict equality check of `action !== null` here will return true and
we'll enter the if-case anyway.

That's quite confusing and was not intended like that, so change the
comparison to a less-strict operator.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3286>
(cherry picked from commit 91ffc4c06bbb0e4c8a5df76642c8576b61fbde9e)