FWNX-1206 Prevent null XmlResolver crash on Linux Pathway

Mono crashes when XmlDocument is told to load a string that
contains URLs if XmlResolver is set to null (which on Windows
means don't try to resolve things). I made a DummyResolver
which resolves all URLs to null. This works at least on Windows.

Change-Id: I13adb9863f289a33eff712a1cbbb2a8a8a10de39