[clang-tidy] Fix bugprone-unchecked-optional-access in TimeSubtractionCheck

Author: zeyi2

clang-tools-extra/clang-tidy/abseil/TimeSubtractionCheck.cpp

@@ -96,6 +96,8 @@ void TimeSubtractionCheck::registerMatchers(MatchFinder *Finder) {
     const std::string TimeInverse = (llvm::Twine("ToUnix") + ScaleName).str();
     std::optional<DurationScale> Scale = getScaleForTimeInverse(TimeInverse);
     assert(Scale && "Unknown scale encountered");
+    if (!Scale)
+      continue;
 
     auto TimeInverseMatcher = callExpr(callee(
         functionDecl(hasName((llvm::Twine("::absl::") + TimeInverse).str()))