if review is empty after edit, `cargo crev review` should abort

[lolbinarycat]

Reproduction steps:

1. run cargo crev review somecratename
2. delete all non-comment lines
3. close the editor

Expected outcome:
stderr: aborted due to empty review
exit status: non-zero

Actual outcome:
empty review is published, verifyable via cargo crev repo query review somecratename