Fix vtkSMTrace segfault when char* is NULL.

Handled by passing a None when char* is NULL.

Change-Id: I256c759a8f4a4b120b6ac042441e76afcf407256
Merge topic 'remove_duplicate_tiff_readers' into next

e1b8c5d3 Fix vtkSMTrace segfault when char* is NULL.