add_test: Restore support for empty test arguments

This was regressed by refactoring in commit e08ba229ee (CMake code rely
on cmList class for CMake lists management (part. 1), 2023-04-14,
v3.27.0-rc1~174^2).  Fix it and add a test case.

Fixes: #24986

Source/cmTestGenerator.cxx

@@ -152,7 +152,8 @@ void cmTestGenerator::GenerateScriptForConfig(std::ostream& os,
     this->EvaluateCommandLineArguments(this->Test->GetCommand(), ge, config),
     // Expand arguments if COMMAND_EXPAND_LISTS is set
     this->Test->GetCommandExpandLists() ? cmList::ExpandElements::Yes
-                                        : cmList::ExpandElements::No
+                                        : cmList::ExpandElements::No,
+    cmList::EmptyElements::Yes
   };
   // Expanding lists on an empty command may have left it empty
   if (argv.empty()) {

Tests/RunCMake/add_test/CheckEmptyArgument.cmake

@@ -0,0 +1,11 @@
+if (NOT CMAKE_ARGV4 STREQUAL "A")
+  message(FATAL_ERROR "wrong parsing of arguments")
+endif()
+
+if (NOT CMAKE_ARGV5 STREQUAL "")
+  message(FATAL_ERROR "Empty argument was dropped")
+endif()
+
+if (NOT CMAKE_ARGV6 STREQUAL "B")
+  message(FATAL_ERROR "wrong parsing of arguments")
+endif()

Tests/RunCMake/add_test/EmptyArgument.cmake

@@ -0,0 +1,5 @@
+enable_testing()
+add_test(
+  NAME "EmptyArgument"
+  COMMAND "${CMAKE_COMMAND}" -P "${CMAKE_CURRENT_LIST_DIR}/CheckEmptyArgument.cmake" -- "A" "" "B"
+)

Tests/RunCMake/add_test/RunCMakeTest.cmake

@@ -33,3 +33,11 @@ foreach(case IN LISTS cases)
   run_case(OLD-${case})
   run_case(NEW-${case})
 endforeach()
+
+block()
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/EmptyArgument-build)
+  run_cmake(EmptyArgument)
+  # Run ctest on the generated CTestTestfile.cmake.
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake_command(EmptyArgument-ctest ${CMAKE_CTEST_COMMAND} -C Debug)
+endblock()