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>

js/ui/keyboard.js

@@ -1513,7 +1513,7 @@ export const Keyboard = GObject.registerClass({
                 });
             }
 
-            if (key.action !== null) {
+            if (key.action) {
                 button.connect('released', () => {
                     if (key.action === 'hide') {
                         this.close(true);