workspace: Properly handle zero spacing in the WorkspaceLayout

It should be allowed to set this._spacing to 0 and thus pass 0 as
rowSpacing or colSpacing to this._adjustSpacingAndPadding(). The current
if-condition there won't add the oversize to the spacing in case 0 is
passed though.

So change that if-condition and explicitely check for null instead.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1698>

js/ui/workspace.js

@@ -463,9 +463,9 @@ var WorkspaceLayout = GObject.registerClass({
         const [topOversize, bottomOversize] = window.chromeHeights();
         const [leftOversize, rightOversize] = window.chromeWidths();
 
-        if (rowSpacing)
+        if (rowSpacing !== null)
             rowSpacing += topOversize + bottomOversize;
-        if (colSpacing)
+        if (colSpacing !== null)
             colSpacing += Math.max(leftOversize, rightOversize);
 
         if (containerBox) {