Invalid UTF-8 paths result in empty JSON files

[ausi]

We should check if the json_encode was successful in

image/src/DeferredImageStorageFilesystem.php

Line 54 in 2dc4390

$this->filesystem->dumpFile($this->getConfigPath($path), json_encode($value));

Related: contao/contao#827 (comment)

[ausi]

See #63