build

refactor: use nullptr #300

Job	Run time
ubuntu cmake	12s
windows (Win32, standard)	1m 23s
macos cmake	15s
ubuntu (g++, standard)	57s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 50s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 3s
windows (Win32, PUGIXML_COMPACT)	2m 1s
ubuntu (g++, PUGIXML_COMPACT)	1m 2s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 40s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	55s
windows (x64, standard)	1m 33s
ubuntu (clang++, standard)	44s
windows (x64, PUGIXML_WCHAR_MODE)	1m 33s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	50s
windows (x64, PUGIXML_COMPACT)	2m 9s
ubuntu (clang++, PUGIXML_COMPACT)	46s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 3s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	40s
macos (clang++, standard)	38s
macos (clang++, PUGIXML_WCHAR_MODE)	1m 33s
macos (clang++, PUGIXML_COMPACT)	37s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	35s
	23m 59s

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