BUG: Fix uninitialized m_Spacing in SignedMaurerDistance

If PrintSelf was called before execution not value had been assigned
to the m_Spacing variable. This defect was detected by Valgrind on the
SimpleITK dashboard.

Change-Id: I206f9c6fb4fb4d524922e3fcf973051f3f165769
BUG: Don't print null bias field

Printing the N4BiasFieldCorrection before execution was causing a
segfault. Print "(null)" when null IVAR is encountered.

Change-Id: I1af252f0734ecd6d524cdd2eb7731a77d096b9cc