Building with alternating --target doesn't invalidate cache

[mischnic]

I found that adding a targets.length > 0 check avoids the exception when running parcel serve --target B: bao5/parcel-error-repro@5d4c8a7

However, the patch seems incomplete since parcel build --target is still broken:

> npx parcel build --target B
√ Built in 1.14s

dist\B\file.js    34 B    305ms
> npx parcel build --target A
√ Built in 1.03s

dist\A\file.js    34 B    198ms
> npx parcel build --target B
√ Built in 429ms

dist\A\file.js    34 B    198ms

The last parcel build --target B ends up building target A. It works properly if you add --no-cache.

Originally posted by @tunafield in #6905 (comment)