build

refactor: use nullptr #302

Job	Run time
ubuntu cmake	14s
ubuntu (g++, standard)	45s
macos cmake	16s
ubuntu (g++, PUGIXML_WCHAR_MODE)	44s
windows (Win32, standard)	1m 2s
ubuntu (g++, PUGIXML_COMPACT)	45s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 6s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	39s
windows (Win32, PUGIXML_COMPACT)	1m 4s
ubuntu (clang++, standard)	36s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 12s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	30s
windows (x64, standard)	1m 55s
ubuntu (clang++, PUGIXML_COMPACT)	34s
windows (x64, PUGIXML_WCHAR_MODE)	1m 48s
windows (x64, PUGIXML_COMPACT)	1m 46s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	31s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 32s
macos (clang++, standard)	1m 31s
macos (clang++, PUGIXML_WCHAR_MODE)	29s
macos (clang++, PUGIXML_COMPACT)	33s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	33s
	20m 5s

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