build

refactor: use nullptr #299

Job	Run time
ubuntu cmake	13s
windows (Win32, standard)	1m 57s
macos cmake	15s
ubuntu (g++, standard)	1m 6s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 23s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 3s
windows (Win32, PUGIXML_COMPACT)	1m 52s
ubuntu (g++, PUGIXML_COMPACT)	1m 5s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 54s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	59s
windows (x64, standard)	1m 31s
ubuntu (clang++, standard)	43s
windows (x64, PUGIXML_WCHAR_MODE)	1m 37s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	45s
windows (x64, PUGIXML_COMPACT)	1m 48s
ubuntu (clang++, PUGIXML_COMPACT)	54s
windows (x64, PUGIXML_NO_EXCEPTIONS)	2m 15s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	40s
macos (clang++, standard)	42s
macos (clang++, PUGIXML_WCHAR_MODE)	49s
macos (clang++, PUGIXML_COMPACT)	42s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	38s
	24m 51s

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