Don't crash if we try to load the config for a MockSystemReader

Instead do nothing.  For now, save() will fail and the config
file is set to null, which may surprise some calling tests.

Change-Id: I1c65f8b1131569da01b4ef33678d813565521fbb
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>