It appears that technically a null format string is not warned under -Wformat-nonliteral, as

the function processing the format string can decided whether or not to accept a null format string (e.g., asl_log).  Fixes <rdar://problem/8269537>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@113469 91177308-0d34-0410-b5e6-96231b3b80d8