Update MailEvent.php

[sylphlv]

Fix issue where if emails are sent asynchronously and an error occurs in the first Invoice that's sent, the next Invoice email will contain PDFs from the first Invoice.

Description

Set $this->parts to default value before attaching PDFs to email.

Manual testing scenarios

1. Set Magento to send emails asynchronously (https://docs.magento.com/user-guide/configuration/sales/sales-emails.html);
2. Enable attaching Invoice PDFs (path: mp_email_attachments/general/is_enabled_attach_pdf)
3. Create first order by replicating issue in Fixing UTF-8 encoding for adress list #18 (for example, use ä (umlaut) in the customer name);
4. Create second order;
5. Invoice first & second orders;
6. Wait for cron to send the emails OR run sales_send_order_invoice_emails cron with https://github.com/netz98/n98-magerun2

Without code changes from this PR, the second Invoice email will contain Invoice PDF from the first invoice.
This PR fixes this by setting $this->parts to the initial value.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)

[sylphlv]

Some priority would be nice on this, as it is a GDPR issue and can cause legal problems for people using this module.

[sylphlv]

@imsamthomas @haitv282 @gixid192 @legendhorus @thaimageplaza
merge this pliz