Issues / question re navl and data conversion

[rgrp]

I ran into a bug with the size field on resources.

• It would not accept an empty value from form (IMO this clearly equates to null/None)
• This could be fixed via using ignore_empty instead of ignore_missing
• However using this means there was no way to empty the field (e.g. i may just want to set the size field back to null not just change to another value)
• similar issues could arise around other fields (such as last_modified ...)
  • cf cset:645031d07b60

To solve this (cset:58acdcfe6d4e) i created an int_converter temporarily in logic/schema.py (this is almost certainly the wrong place). But I think it raises a bigger issue about the conversion layer and how it works.