gh-138568: Make help mode in PyREPL not exit on empty line input

[ambv]

This revives gh-138569 that was automatically closed when the original PR's author @yihong0618 deleted their cpython repository fork.

• Issue: The exit behavior is not the same as the doc help mode in repl #138568

[yihong0618]

so sorry for that...

[ambv]

Don't worry about it, @yihong0618. As you can see, it wasn't hard to recover your work.

[miss-islington-app]

Thanks @ambv for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @ambv for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @ambv, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b3e4a3462f6011cacdfc58c565c69e4d7de6b9cd 3.13

[bedevere-app]

GH-143519 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-143520 is a backport of this pull request to the 3.13 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot ARM64 macOS 3.14 (tier-2) has failed when building commit 63f8a86.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1772/builds/773) and take a look at the build logs.
4. Check if the failure is related to this commit (63f8a86) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1772/builds/773

Failed tests:

• test_urllib2net

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 12, done.        
remote: Counting objects:  12% (1/8)        
remote: Counting objects:  25% (2/8)        
remote: Counting objects:  37% (3/8)        
remote: Counting objects:  50% (4/8)        
remote: Counting objects:  62% (5/8)        
remote: Counting objects:  75% (6/8)        
remote: Counting objects:  87% (7/8)        
remote: Counting objects: 100% (8/8)        
remote: Counting objects: 100% (8/8), done.        
remote: Total 12 (delta 7), reused 7 (delta 7), pack-reused 4 (from 2)        
From https://github.com/python/cpython
 * branch                    3.14       -> FETCH_HEAD
Note: switching to '63f8a863d15d228bdefc6082b2772031c1a0c959'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 63f8a863d15 [3.14] gh-138568: Make `help` mode in PyREPL not exit on empty line input (GH-143512) (GH-143519)
Switched to and reset branch '3.14'

make: *** [buildbottest] Error 2

[ambv]

The buildbot failure's unrelated. It's a "Resource 'ftp://www.pythontest.net/' is not available" error.