fix(@angular/build): treat empty browsers array as undefined in unit-… #2817

Job	Run time
build	22s
build-e2e-windows	20s
lint	2m 12s
e2e-windows	0s
test	-1s
Browser Compatibility Tests	-1s
publish-snapshots	-1s
e2e	-1s
e2e-snapshots	-1s
e2e-package-managers	-1s
	2m 48s

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