IPC: set ws reply "num" member to -1 when named

When a named workspace (i.e., a workspace that has a name that does not
begin with text that can be parsed as an integer greater than or equal
to zero) is represented by the ipc as a workspace json object such as
can be queried with `i3-msg -t get_workspaces`, set the num property to
-1 instead of json null.

This is for convenience of ipc consumers using type-constrained
languages such as C which have difficulty cleanly expressing nullable
integers.

fixes #1368

src/ipc.c

@@ -617,10 +617,7 @@ IPC_HANDLER(get_workspaces) {
             y(map_open);
 
             ystr("num");
-            if (ws->num == -1)
-                y(null);
-            else
-                y(integer, ws->num);
+            y(integer, ws->num);
 
             ystr("name");
             ystr(ws->name);