guard against null strings with non-zero size #162

Job	Run time
build-ubuntu (llvm-mingw-oak-release, llvm-mingw)	1m 39s
build-ubuntu (llvm-mingw-willow-release, llvm-mingw)	1m 28s
build-ubuntu (clang-cross-willow-release, clang-cross)	3m 21s
build-ubuntu (mingw-oak-release, mingw)	1m 44s
build-ubuntu (mingw-willow-release, mingw)	2m 14s
build-windows (clang-willow-release)	2m 46s
build-windows (msvc-willow-release)	1m 30s
build-windows (msvc-oak-release)	1m 14s
clang-format	1m 33s
clang-tidy (clang-oak-release)	11m 44s
build-windows (clang-oak-release)	3m 11s
clang-tidy (clang-willow-release)	11m 19s
spelling	7s
build-ubuntu (clang-cross-oak-release, clang-cross)	3m 17s
	47m 7s

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