take care of non-existant old workspaces

docs/ipc

@@ -646,7 +646,9 @@ This event consists of a single serialized map containing a property
 +change (string)+ which indicates the type of the change ("focus", "init",
 "empty", "urgent").  Additionally, when the change is "focus", an +old
 (object)+ and a +current (object)+ properties will be present with the
-previous and current workspace respectively.
+previous and current workspace respectively.  When the first switch
+occurs (when i3 focuses the workspace visible at the beginning) there is
+no previous workspace, and the +old+ property will be set to +null+.
 
 *Example:*
 ---------------------

src/workspace.c

@@ -350,6 +350,9 @@ static void _workspace_focus_event(Con *current, Con *old) {
     dump_node(gen, current, false);
 
     ystr("old");
+    if (old == NULL)
+        y(null);
+    else
         dump_node(gen, old, false);
 
     y(map_close);