Allow users to skip "arc:prompt" by entering no text

Summary:
Fixes T9476. Currently, if you enter no text in "arc:prompt", we abort resolution immediately.

However, this is a bit inconsistent (other rules that fail to resolve are skipped) it is reasonable to put some default rule behind "arc:prompt" so that you can just mash enter to select it. This construction is unusual, but seems fine and sensible to me.

If you're using "arc:prompt" as the last rule, the behavior is the same as before, so this should only make the rule more useful.

Test Plan:

• Ran arc which --base 'arc:prompt, git:HEAD^' with and without the patch.
• Without the patch, entering no text at "arc:prompt" failed immediately.
• With the patch, entering no text caused fallback to the "git:HEAD^" rule.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9476

Differential Revision: https://secure.phabricator.com/D14187