JAMES-2434 IsFromMailingList matcher

[Maxxx873]

• Implemented IsFromMailingList matcher with using AutomaticallySentMailDetector::isMailingList utility. This matcher returns recipients of the mail if the mail is from a mailing list.
• Fixed IsSingleRecipient matcher - for methods that return an array, prefer returning an empty array over a null value.

[chibenwa]

2 licenses to add but overall thanks for this great contribution!

[chibenwa]

You are missing the compulsary Apache V2 license header comment in this file.

[chibenwa]

You are missing the compulsary Apache V2 license header comment in this file.

[Maxxx873]

2 licenses to add but overall thanks for this great contribution!

Thank you for taking the time to review my code. I appreciate your feedback and will use it to improve my work in the future.

[Arsnael]

Thank you for the nice contribution @Maxxx873 , the code looks good to me :)

[Arsnael]

20:46:20,494 [ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.1.2:check (check-style) on project apache-mailet-standard: You have 2 Checkstyle violations. -> [Help 1]

There is some checkstyle issue with your code (we have a maven plugin doing some strict code style checking following this file: https://github.com/apache/james-project/blob/master/checkstyle.xml during the build)

I would guess likely some unused imports in the files in your PR that you added/modified. Please help fixing this :)

[Maxxx873]

20:46:20,494 [ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.1.2:check (check-style) on project apache-mailet-standard: You have 2 Checkstyle violations. -> [Help 1]

There is some checkstyle issue with your code (we have a maven plugin doing some strict code style checking following this file: https://github.com/apache/james-project/blob/master/checkstyle.xml during the build)

I would guess likely some unused imports in the files in your PR that you added/modified. Please help fixing this :)

Sorry, I'm just used to a different import order :)

[Arsnael]

Sorry, I'm just used to a different import order :)

No worries :)