Warning: imagecreatefromstring(): Data is not in a recognized format

Problem/Motivation

In some cases when in the flmngr or flmngr-cache folders exists broken files, after every image upload from the media library we will get the next warning
Warning: imagecreatefromstring(): Data is not in a recognized format in Drupal\n1ed\Flmngr\fs\CachedFile->getPreview() (line 132 of modules/contrib/n1ed/src/Flmngr/fs/CachedFile.php).
It happens because variable $contents which we pass into imagecreatefromstring($contents); could be empty.

Steps to reproduce

1. open content entity edit page;
2. attach existing image using media “Browse” button;
3. scroll to the beginning of this field (if it has many images).

Proposed resolution

So I'm propose to check whether variable $contents empty or not before creating an image.
Patch with fix would be attached in the first comment.

Remaining tasks

User interface changes

API changes

Data model changes