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
Merge topic 'FixN4BiasFieldPrintNullField'

113a1b0 BUG: Don't print null bias field