Fix crash when updating a list of files

When using JackRabbit, since NXP-1085, updating list of files may crash.

Steps to reproduce:

• use nxu:inputList with diff="false" on a document files layout/page
• add two attached files on a document
• navigate elsewhere
• come back on the document page
• edit just one of the files

Ends up with a jcr error, due to persist() being called on a LazyBlob that's never been initialized (null input stream)

=> need to fix LazyBlob#persist() and make sure that the list widget takes the widget 'diff' property into account