window-x11: Focus a window in the active workspace as take-focus fallback
Starting with commit 2db94e2e we try to focus a fallback default focus window if no take-focus window candidate gets the input focus when we request it and we limit the focus candidates to the current window's workspace.
However, if the window is unmanaging, the workspace might be unset, and we could end up in deferencing a NULL pointer causing a crash.
So, in case the window's workspace is unset, just use the currently active workspace.
Closes #687 (closed)