ValueError: Path cannot be empty in file_get_contents() (line 611 of modules/contrib/swiftmailer/src/Plugin/Mail/SwiftMailer.php).

Problem/Motivation

I'm getting
ValueError: Path cannot be empty in file_get_contents() (line 611 of modules/contrib/swiftmailer/src/Plugin/Mail/SwiftMailer.php).
when trying to send an email, for example password reset

Steps to reproduce

Proposed resolution

First check, if the value passed is empty, before running the method.

Remaining tasks

User interface changes

API changes

Data model changes