chore(core): enable eslint/strict-boolean-expressions & strictNullChecks #1973

Job	Run time
Check version consistency of packages	10s
Check coding style	18s
eslint	2m 26s
setup-and-test (ubuntu-latest, 18.x)	2m 28s
setup-and-test (ubuntu-latest, 20.x)	2m 42s
	8m 4s

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