[clang-format] Fix -Wconversion-null warning in GCC

GCC -Wconversion-null warning appeared after 9a63380260860b657b72f07c4f0e61e382ab934a.
There was a similar problem already in the past:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20131230/096230.html

Patch committed on behalf of @dendibakh

Differential Revision: https://reviews.llvm.org/D60726

llvm-svn: 358441