Bug 26781: Check for subfield defined rather than truth

Bug 26781: Check for subfield defined rather than truth

To recreate:
 1 - Have a record with 100$0 defined
 2 - Define a Marc Modification template:
    Delete field 100$0
 3 - Perform Batch record modification, enter the record above in the list of records
 4 - Preview the MARC
 5 - Note the entire 100 field has been deleted, don't modify the record
 6 - Apply patch
 7 - Restart all the things
 8 - Test the modification again
 9 - This time the preview confirms only the subfield 0 deleted
10 - Modify the record
11 - Confirm the record is correctly updated

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
JD amended patch: Fix typo ony vs only

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit be4f0c7639a679b1ec808e6e9cf9af0fc9576e79)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>