[clang-tidy] Fix a crash for raw-string-literal check.

getSourceText could return an empty string for error cases (e.g. invalid
source locaiton), this patch makes the code more robust.

The crash did happen in our internal codebase, but unfortunately I
didn't manage to get a reproduce case. One thing I can confirm from
the core dump is that the crash is caused by calling isRawStringLiteral
on an empty Text.

Differential Revision: https://reviews.llvm.org/D102770