Print NULL as "(null)" in diagnostic message

Summary:
Passing a null pointer to the printf family for a %s format specifier leads to undefined behaviour. The tests currently expect (null). Explicitly test for a null pointer and provide the expected string.

Authored By: andusy

Reviewers: hubert.reinterpretcast, xingxue, jasonliu, daltenty, cebowleratibm

Reviewed By: hubert.reinterpretcast

Subscribers: arphaman, jsji, cfe-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63786

llvm-svn: 364462