Attached file not removed when title is emptied

Problem/Motivation

When a file is attached to a node using the CMIS Field, it must be removed on node update if the title is emptied.
Instead, if the title is emptied and the node is saved, the file remains attached to the node but with an empty title.

Proposed resolution

The problem is in the implementation of hook_field_is_empty(). The function checks if $item['path'] is empty; instead, it must check $item['title'].