build

refactor: use nullptr #298

Job	Run time
ubuntu cmake	12s
windows (Win32, standard)	1m 17s
macos cmake	17s
ubuntu (g++, standard)	23s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 18s
ubuntu (g++, PUGIXML_WCHAR_MODE)	25s
windows (Win32, PUGIXML_COMPACT)	1m 34s
ubuntu (g++, PUGIXML_COMPACT)	23s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 33s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	24s
windows (x64, standard)	1m 48s
windows (x64, PUGIXML_WCHAR_MODE)	1m 9s
ubuntu (clang++, standard)	20s
windows (x64, PUGIXML_COMPACT)	1m 25s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	19s
ubuntu (clang++, PUGIXML_COMPACT)	27s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 29s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	18s
macos (clang++, standard)	30s
macos (clang++, PUGIXML_WCHAR_MODE)	34s
macos (clang++, PUGIXML_COMPACT)	34s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	29s
	17m 8s

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