NSX: do not raise on missing router during migration step

Return None for missing router rather raising NotFound exception;
this is what the client code was expected to handle in the first
place.

Closes-bug: 1313918

Change-Id: I573770a69d0170b027fc990c29aaa87b1fc8eccc
(cherry picked from commit 5e843850527ede7a56fcc24753f87c1e9e3e8bed)