Fix CMakeLists.txt syntax error - properly nested if statements

- Fixed malformed if/endif structure
- Removed duplicate if(EXISTING_SOURCES) check

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-benchmark/tests/CMakeLists.txt

@@ -28,7 +28,6 @@ if(BUILD_TESTING)
     if(MIGRATION_TEST_SOURCES)
         set(EXISTING_SOURCES ${MIGRATION_TEST_SOURCES})
 
-    if(EXISTING_SOURCES)
         add_executable(migration_tests ${EXISTING_SOURCES})
         
         target_compile_features(migration_tests PRIVATE cxx_std_23)
@@ -67,7 +66,6 @@ if(BUILD_TESTING)
         gtest_discover_tests(migration_tests)
         
         message(STATUS "Migration tests enabled: ${EXISTING_SOURCES}")
-    else()
-        message(WARNING "No migration test sources found in ${CMAKE_CURRENT_SOURCE_DIR}")
-    endif()
+else()
+    message(WARNING "No migration test sources found in ${CMAKE_CURRENT_SOURCE_DIR}")
 endif()