[FIX] mail: removing whitespace on message_id from incoming mail

Because of a whitespace character present at the beginning of the message_id of some mails,
they were fetched twice.

From the SQL side, we see that Outlook is formatting the header following the RFC822
by allowing the header to look like:

Header:<CRLF>
<whitespace><hash_of_msg_id>

From RFC822:
"Each header field can be viewed as a single, logical line of ASCII characters.
For convenience, the field-body portion of this conceptual entity can be split
into a multiple-line representation" (abr.)

It was also possible to find out by checking the logs.
One or two whitespace(s) can be seen before the hash of the msg_id.

The following commit is making sure we use and save the message_id without the whitespace
by stripping them off on the coming mail.

OPW-2006806

Applying CHS solution to avoid crash on empty header.

closes odoo/odoo#35682

Signed-off-by: bve-odoo <Abridbus@users.noreply.github.com>