[BUGFIX] Correctly check decoded content

On publishing a new extension version, the content of each
file is checked against the calculated MD5 checksum. If the
file content is however an empty string, this check previously
failed. Therefore, the `$decodedContent` is now strictly
checked for `false`.
10 jobs for 504-error-md5-does-not-match-for-file-classes-on-version-upload in 6 minutes and 43 seconds (queued for 3 seconds)