ENH: Define print of null smart pointers

Print "(null)" instead of segfaulting. While this is different than a
raw pointers behavior, this is much more user friendly.

Change-Id: I3d311f0e6d575d7def44b34e5c0cc42f066d52c3
Merge topic 'PointersPrintNull'

3c63779 ENH: Define print of null smart pointers