Ensure active window isn't focused when screen is locked

If the screen is locked, but the lockscreen isn't ready for painting,
pickFocus() returns the active window. This somehow leads to the
meta key getting "stuck" if you lock the screen with Meta+L.

BUG: 495325


(cherry picked from commit c1e3dc0e08)

Co-authored-by: Adam Nydahl <loginer@loginer.net>

src/keyboard_input.cpp

@@ -209,6 +209,7 @@ Window *KeyboardInputRedirection::pickFocus() const
                 return t;
             } while (it != stacking.begin());
         }
+        return nullptr;
     }
 
     if (input()->isSelectingWindow()) {