Local file returns as is_file_readable_externally_by_hash() == true if preferexternal is set and file is 0 bytes in size

[petersistrom]

If a local file has a size of 0 bytes and the tool_objectfs | preferexternal setting is turned on, then the is_file_readable_externally_by_hash

moodle-tool_objectfs/classes/local/store/object_file_system.php

Line 249 in 6388430

if ($contenthash === sha1('')) {

will still return true and the plugin will look for a non-existent external file and return a 404, even though the file exists locally.

[petersistrom]

Related to #609