[14.0][FIX] crm: check if email_from is defined

[remi-filament]

Description of the issue/feature this PR addresses:
Facing a special case with both CRM and mail_tracking (from OCA social repo) installed.
When an opportunity or a lead was created from an incoming e-mail and then you remove the email_from on opportunity/lead (for instance because you have forwarded an email to Odoo to create opportunity but you do not want to keep your own email address as email_from for the lead), you get an error that boolean does not have lower() method.
This issue has been introduced with odoo@9ef715f part of odoo#74474

Since mail_tracking is trying to suggest extra emails, you get all message_ids.email_to as emails parameter when calling _message_partner_info_from_emails(emails) which it tries to compare with crm_lead.email_from

This PR fixes this issue by checking that email and self.email_from exist before trying to lower() them (as it was done before the above commit)

Current behavior before PR:
Stack trace when accessing lead / opportunity created from incoming e-mail where email_from has been emptied.

Desired behavior after PR is merged:
No more error, working correctly

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I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖