Use #parts instead of #part to read all the parts of the Message.

What does this MR do?

Fix an incorrect usage of a method to read the parts of an email message.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It was adding an extra empty Mime section which was giving issues in some mail clients. You can see more information related to this here: https://productforums.google.com/forum/#!topic/gmail/aVoLmVuhV_c

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26463 (closed)