Move container to marked workspace: refine corner case

This commit should fix "move con to parent" trick (see below) in the
case when con->parent->parent is a workspace.

The trick:

    mark _a, focus parent, focus parent, mark _b,
    [con_mark=_a] move window to mark _b, [con_mark=_a] focus

The trick got broken in commit 626af81232
in order to fix an i3 crash (#2003).  Reverting said commit fixes the
trick.  The crash is caused by the fact that empty workspace isn't
considered a split (checked in src/con.c:1324), so the moved window ends
up as a sibling of the target workspace, not as its child.

testcases/lib/i3test.pm.in

@@ -1230,7 +1230,7 @@ sub create_layout {
 
             $r = $r . '{"swallows": [{';
             $r = $r . '"class": "^' . "$char" . '$"';
-            $r = $r . '}]},';
+            $r = $r . '}]}' . ($depth == 0 ? "\n" : ',');
         } else {
             die "Could not understand $char";
         }