The default configuration of libinput-gestures utility invokes wmctrl to
switch between desktops. It uses wmctrl because this works on both Xorg
and Wayland (via XWayland). Unfortunately, this generates the following
warning message every time, in both Xorg and Wayland desktops:

"Received a NET_CURRENT_DESKTOP message from a broken (outdated) client
who sent a 0 timestamp"

The desktop switch still works fine. The tiny code change here removes
this specific warning because, as the prefacing code comment originally
said and still says, older clients can validly pass a 0 time value so
why complain about that?

I also refactored the "if (workspace)" code slightly to avoid the double
test of the workspace value.

Based on mutter commit:
mutter@e9cc220c