Fix GCC warning by specializing PdmVariantEqualImpl for QString #23084

Job	Run time
build (Release, Windows Latest MSVC, windows-2022, cl, cl, true, true, true, true, false, bootstr...	34m 7s
build (Release, Ubuntu 24.04 clang-19, ubuntu-24.04, clang-19, clang++-19, true, true, false, fal...	2h 38m 10s
build (Release, Windows_2022_qt_610, windows-2022, cl, cl, false, true, false, true, false, boots...	33m 32s
build (Release, Ubuntu 24.04 gcc, ubuntu-24.04, gcc-14, g++-14, true, true, true, false, true, bo...	2h 7m 16s
Upload release to PyPI	-1s
	5h 53m 4s

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