output-xrandr: Don't treat 0 as invalid backlight value

Whether a value is in range depends on the backlight-min/max values,
and I didn't spot anything that excludes 0 as a valid lower limit
outright.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2947>

src/backends/x11/meta-output-xrandr.c

@@ -529,7 +529,7 @@ output_get_backlight_xrandr (MetaOutput *output)
     return -1;
 
   value = ((int*)buffer)[0];
-  if (value > 0)
+  if (value >= 0)
     return normalize_backlight (output, value);
   else
     return -1;