Generalize lemma `null_set_integral` #394

Job	Run time
coq	55s
mathcomp-classical	2m 52s
mathcomp	51s
mathcomp-analysis-single	16m 43s
mathcomp-reals	2m 14s
mathcomp-experimental-reals	2m 14s
mathcomp-reals-stdlib	1m 42s
mathcomp-analysis	14m 21s
mathcomp-analysis-stdlib	2m 33s
ssprove	7m 21s
	51m 46s

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