Adam,
it happens because while uploading the signature property we convert it into different format based on the SAP database needs. while converting if the data is blank as in your case the converted data might append zeros. which will make the file corrupted. So, while uploading you have to handle this appending zeros case.
Here is my question if your signature property is blank why do you want to send the data to database? you shouldn't.
This is an exceptional scenario which you have to handle either in Agentry or in Java.
Hope this makes you clear.
Thanks
Ajitesh