[clang-tidy] Fix bugprone-bad-signal-to-kill-thread crash when `SIGTERM` was...

[clang-tidy] Fix bugprone-bad-signal-to-kill-thread crash when `SIGTERM` was undefined after definition.

`PP->getMacroInfo()` returns nullptr for undefined macro, which leads to null-dereference at `MI->tockens().back()`.
Stack dump:
```
 #0 0x000000000217d15a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/llvm-project/build/bin/clang-tidy+0x217d15a)
 #1 0x000000000217b17c llvm::sys::RunSignalHandlers() (/llvm-project/build/bin/clang-tidy+0x217b17c)
 #2 0x000000000217b2e3 SignalHandler(int) (/llvm-project/build/bin/clang-tidy+0x217b2e3)
 #3 0x00007f39be5b1390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
 #4 0x0000000000593532 clang::tidy::bugprone::BadSignalToKillThreadCheck::check(clang::ast_matchers::MatchFinder::MatchResult const&) (/llvm-project/build/bin/clang-tidy+0x593532)
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85401