Deprecated function: preg_split(): Passing null to parameter #2 ($subject) of type string is deprecated in pet_parse_mails() (line 55 of /app/web/modules/composer/pet/pet.module)

Problem/Motivation

When function pet_parse_mails($mail_text) receives NULL in $mail_text, PHP throws a warning like this:

Deprecated function: preg_split(): Passing null to parameter #2 ($subject) of type string is deprecated in pet_parse_mails() (line 55 of /app/web/modules/composer/pet/pet.module)

Steps to reproduce

- Create a PET
- Trigger an email using a PET without a cc and without a bcc

Proposed resolution

If $mail_text is not a valid string, early return an empty array.

Remaining tasks

Make patch.