When storing an object, empty flexible fields are removed from the xxx_a...

...ttributes DB table.

There was no way to remove empty (useless) fields in item_attributes/album_attributes tables (these two tables keep growing endlessly).
In the proposed modification, I consider that any empty flexible field should not exist in the DB.

Existing empty flexible fields (of all objects) are not cleaned yet. For my part, I ran two SQL requests manually on the DB:
remove from item_attributes where value='';
remove from album_attributes where value='';
There should be a way to integrate these requests in the code.