Update string unittests to test valid/invalid nullptr #30

Job	Run time
android-armeabi-v7a-ndk-cmake	5m 52s
android-arm64-v8a-ndk-latest-cmake	7m 15s
android-arm64-v8a-ndk-cmake	5m 46s
linux-gcc-cmake	40m 35s
linux-gcc-make	44m 55s
linux-gcc-make-armv7l	13m 36s
linux-gcc-make-asan	50m 59s
linux-gcc-make-cxx20	48m 22s
linux-gcc-make-asan-no-soo	50m 44s
linux-gcc-make-tsan	51m 14s
linux-gcc-make-ubsan	57m 43s
linux-emscripten-cmake	4m 45s
linux-gcc-make-mysql	7m 14s
linux-gcc-make-cross-armhf	13m 43s
linux-gcc-make-postgres	6m 51s
linux-gcc-make-redis	6m 56s
macos-clang-cmake-openssl3-visibility-hidden	50m 27s
macos-clang-cmake-openssl	51m 47s
macos-clang-cmake-openssl3	49m 20s
macos-clang-make	50m 16s
macos-clang-make-openssl3-asan	53m 36s
macos-clang-make-openssl3-tsan	16m 1s
macos-clang-make-openssl3-ubsan	16m 7s
macos-clang-make-visibility-hidden	16m 15s
windows-2022-msvc-buildwin-x64	34m 22s
windows-2022-msvc-cmake	33m 15s
linux-gcc-make-mongodb	7m 2s
linux-gcc-make-odbc	8m 36s
linux-gcc-make-sqlite-no-sqlparser	7m 20s
	13h 30m 54s

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