`@bors r=` should reject the empty/whitespace reviewer

[jieyouxu]

[jieyouxu]

FWIW the invocation was (␣ is used to represent a whitespace)

@bors r=␣Kobzol rollup

Technically that could be trimmed, but rejecting it outright seems way safer.

[fmease]

Isn't that a homu issue tho, not a bors(2) one?

(edit: I mean maybe bors(2) has the same problem/oversight coincidentally, idk)

[jieyouxu]

Yes, this is meant for new bors in case it has the same behavior, as I don't think it's worth bothering to fix this in old homu given redeploy is scary :D

[Kobzol]

We already handled this, but I added a test in #354, for good measure.

[jieyouxu]

Oh cool!