Handle attachments without name in content-type header

[romsahel]

The case was encountered in production.

Fixes the error below for emls without a name property in the Content-Type header:
e.g.:

Content-Type: application/pdf;
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="20250410-1947.pdf"

The error:

     The following arguments were given to Mail.Parsers.RFC2822.parse_encoded_word/2:

         # 1
         ["application/pdf"]

         # 2
         []

     Attempted function clauses (showing 3 out of 3):
         defp parse_encoded_word("", _opts)
         defp parse_encoded_word(<<"=?", value::binary>>, opts)
         defp parse_encoded_word(<<char::utf8, rest::binary>>, opts)