Bug 32456: Preserve date accessioned if a date is not explicitly passed

Bug 32456: Preserve date accessioned if a date is not explicitly passed

To test:
 1 - Edit an item record to have a blank date accessioned
 2 - Save the record as marcxml from the detail page
 3 - Edit the item to have an accession date
 4 - Stage and import the file - matching in KohaBiblio and replacing items
 5 - Import the file
 6 - Confirm the date accessioned is cleared
 7 - Apply patch
 8 - Provide an accession date for the item
 9 - Stage and import the file again
10 - Confirm the date remained
11 - Edit the item to remove the date
12 - Confirm you can manually blank the field

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>