Bug 32797: Check if fields have a value, not if true

Bug 32797: Check if fields have a value, not if true

When saving OAI set mappgins we are checking for truth, this prevents
saving subfield 0. We should check for content

To test:
1 - Go to Admin->OAI sets
2 - Create a set
3 - Define mappings for the set
4 - Attempt to save a rule on field 952 subfield 0 (withdrawn)
5 - It does not save
6 - Apply patch
7 - Try again
8 - Success!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>