Bug 21406: [18.11.x only] Prevent errors from undefined fields in requests

Bug 21406: [18.11.x only] Prevent errors from undefined fields in requests

To test:
1 - In 18.11 enable ILL and install the freeform backend
2 - Create a request for 'Journal article'
3 - Note there is no 'Author' field
4 - Try to view ILL requests
5 - You get a datatables error about missing metadata_author column
6 - Apply patch
7 - No more error

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>