pants help-all errors on missing help text, but help text is not a required field.

[TansyArron]

Describe the bug
A clear and concise description of the bug.
When running pants help-all on my own repo I get:

$ pants help-all
11:11:51.53 [INFO] Initializing scheduler...
11:11:55.85 [INFO] Scheduler initialized.
11:11:56.19 [ERROR] type object 'MyCustomField' has no attribute 'help'


Use --print-stacktrace for more error details and/or -ldebug for more logs. 
See https://www.pantsbuild.org/v2.19/docs/troubleshooting for common issues.
Consider reaching out for help: https://www.pantsbuild.org/v2.19/docs/getting-help

I would have expected either:

• help is a required parameter and pants help-all fails with the above message.
• help is not a required parameter, and pants help-all does not error.

Pants version
Which version of Pants are you using?
2.19.1

OS
Are you encountering the bug on MacOS, Linux, or both?
Linux

Additional info
Add any other information about the problem here, such as attachments or links to gists, if relevant.
Super minor issue, and I've fixed it by adding help text, but it seemed like the sort of inconsistency we should fix.

[sureshjoshi]

What does MyCustomField inherit from? Because help is (supposed to be) a required field