Fix Enumerable#each block return type

[straight-shoota]

The block's return type restriction should be empty instead of underscore (see #10467 (comment)). There's an unintended subtle difference for yielding blocks and underscore may break perfectly valid code (https://app.circleci.com/pipelines/github/straight-shoota/crinja/743/workflows/6a3278a5-8635-417a-96b5-ff7a483ae983/jobs/1307).

The block return type on Enumerable#each itself isn't really important because the block param type restriction is pretty much ignored for abstract defs (see #10902). But the type restriction on Iterator#each matters.

/cc #10905 (comment)

[asterite]

An alternatively could be to remove the type restrictions.

[straight-shoota]

Yes, I mentioned that in #10905 (comment)
But the type restriction on Iterator#each was added because it helps identify bugs. So I'd prefer to keep it. With nil return type, everything should be fine. 🤞

[straight-shoota]

Despite #10933 being merged, I think this is still a good change because the output type of Enumerable#each's block is not used (see #10931 (comment)).