EDTFField fails to re-save fields without natural text field value

[jmurty]

If the EDTFField value is set on a model instance and the instance is subsequently re-saved with an empty natural_text_field value, the pre_save method fails because it tries to re-parse the existing EDTF field python value which is not a string.

[jmurty]

This is currently an issue for AGSA in particular because we are deliberately setting just the EDTF field on works without setting the corresponding natural_text_field value at all, which is backwards from the way the field wants to be used.