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

docs/ipc

@@ -156,7 +156,7 @@ following properties:
 
 num (integer)::
 	The logical number of the workspace. Corresponds to the command
-	to switch to this workspace.
+	to switch to this workspace. For named workspaces, this will be -1.
 name (string)::
 	The name of this workspace (by default num+1), as changed by the
 	user. Encoded in UTF-8.