Update string unittests to test valid/invalid nullptr #32

Job	Run time
android-arm64-v8a-ndk-cmake	6m 4s
android-armeabi-v7a-ndk-cmake	5m 48s
linux-gcc-make-armv7l	14m 8s
android-arm64-v8a-ndk-latest-cmake	7m 52s
linux-emscripten-cmake	5m 10s
linux-gcc-cmake	40m 25s
linux-gcc-make-asan	51m 26s
linux-gcc-make	44m 31s
linux-gcc-make-asan-no-soo	50m 22s
linux-gcc-make-cross-armhf	14m 18s
linux-gcc-make-cxx20	48m 20s
linux-gcc-make-mysql	6m 57s
linux-gcc-make-odbc	8m 32s
linux-gcc-make-postgres	6m 55s
linux-gcc-make-mongodb	6m 59s
linux-gcc-make-redis	7m 10s
linux-gcc-make-sqlite-no-sqlparser	7m 13s
linux-gcc-make-tsan	51m 5s
linux-gcc-make-ubsan	57m 35s
macos-clang-cmake-openssl	1h 9m 16s
macos-clang-cmake-openssl3	50m 32s
macos-clang-cmake-openssl3-visibility-hidden	50m 16s
macos-clang-make	51m 4s
macos-clang-make-openssl3-asan	54m 26s
macos-clang-make-openssl3-tsan	52m 50s
macos-clang-make-openssl3-ubsan	1h 1m 50s
macos-clang-make-visibility-hidden	49m 53s
windows-2022-msvc-buildwin-x64	35m 26s
windows-2022-msvc-cmake	34m 48s
	15h 51m 11s

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