[ticket/11551] Fix error in system tab if get_remote_file returns empty string

Currently a debug error is thrown. With this patch, it fails without keeping
the admin from being able to access the rest of the System modules by showing
a nicer error "Failed to obtain latest version information."

PHPBB3-11551
Merge remote-tracking branch 'remotes/imkingdavid/ticket/11551' into develop-olympus

# By David King
# Via David King
* remotes/imkingdavid/ticket/11551:
  [ticket/11551] Fix error in system tab if get_remote_file returns empty string