Explicitly include NULL as a value that tagQueryFindDescendants_() sh… #692

Job	Run time
R-CMD-check / setup	33s
website / pkgdown	2m 50s
routine / routine	2m 55s
R-CMD-check / macOS-latest (4.3.1)	5m 12s
R-CMD-check / windows-latest (devel)	5m 51s
R-CMD-check / windows-latest (4.3.1)	11m 27s
R-CMD-check / windows-latest (4.1)	9m 41s
R-CMD-check / windows-latest (3.6)	7m 1s
R-CMD-check / ubuntu-20.04 (devel)	2m 57s
R-CMD-check / ubuntu-20.04 (4.3.1)	3m 16s
R-CMD-check / ubuntu-20.04 (4.2.3)	3m 29s
R-CMD-check / ubuntu-20.04 (4.1.3)	3m 58s
R-CMD-check / ubuntu-20.04 (4.0.5)	3m 15s
R-CMD-check / ubuntu-20.04 (3.6.3)	2m 57s
	1h 5m 22s

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