Linting: Fix empty evaluated genex

Update logic added by commit 099934e313 (Add generator expression
support to static code analysis hooks, 2023-03-24, v3.27.0-rc1~261^2),
and preserved by commit 993dde925f (TargetGenerator: Factor out
generation of code check rules, 2023-05-12, v3.27.0-rc1~84^2~2), to
handle a generator expression that evaluates to the empty string.

If `<LANG>_CPPCHECK`, `<LANG>_CPPLINT`, `<LANG>_CLANG_TIDY`, or
`<LANG>_INCLUDE_WHAT_YOU_USE` are empty after evaluating generator
expressions, do not run their lints.

Fixes: #25265

Source/cmCommonTargetGenerator.cxx

@@ -335,10 +335,7 @@ std::string cmCommonTargetGenerator::GenerateCodeCheckRules(
     auto evaluatedProp = cmGeneratorExpression::Evaluate(
       *value, this->GeneratorTarget->GetLocalGenerator(), config,
       this->GeneratorTarget, nullptr, this->GeneratorTarget, lang);
-    if (!evaluatedProp.empty()) {
-      return evaluatedProp;
-    }
-    return *value;
+    return evaluatedProp;
   };
   std::string const tidy_prop = cmStrCat(lang, "_CLANG_TIDY");
   tidy = evaluateProp(tidy_prop);

Tests/RunCMake/MultiLint/RunCMakeTest.cmake

@@ -24,6 +24,7 @@ run_multilint(CXX)
 if(NOT RunCMake_GENERATOR STREQUAL "Watcom WMake")
   run_multilint(C-launch)
   run_multilint(CXX-launch)
+  run_multilint(genex)
 endif()
 
 function(run_skip_linting test_name)

Tests/RunCMake/MultiLint/genex.cmake

@@ -0,0 +1,6 @@
+enable_language(CXX)
+set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE "$<IF:$<BOOL:FALSE>,${PSEUDO_IWYU},>")
+set(CMAKE_CXX_CLANG_TIDY "$<IF:$<BOOL:FALSE>,${PSEUDO_TIDY} --error,>")
+set(CMAKE_CXX_CPPLINT "$<IF:$<BOOL:FALSE>,${PSEUDO_CPPLINT} --error,>")
+set(CMAKE_CXX_CPPCHECK "$<IF:$<BOOL:FALSE>,${PSEUDO_CPPCHECK} -bad,>")
+add_executable(main main.cxx)