Bug 28290: Don't send subfields to 'as_string' if none to send

Bug 28290: Don't send subfields to 'as_string' if none to send

This fixes an error where we send a blank string to
Marc::Field->as_string

The subroutine fails as the regex is marked as incomplete as there is
nothing between the brackets

Also fixes a missing hidden input for 'item_action' when no items are
staged during import

 1 - Define a new matching rule with no subfields
    Match-threshold: 1000
    Record type: bibliographic record
    Search-index: Other-control-number
    Score: 1000
    Tag: 035
    Subfields:
    Offset:
    Length:
 2 - Attempt to stage a record with an 035 using this rule (i will
     attach example to bug)
 3 - Staging fails
 4 - Stage again with no matching rule
 5 - Manage the staged record, try to apply the matching rule
 6 - It fails, 500 error
 7 - Apply patch
 8 - Restart all the things
 9 - Apply rule again, it works!
10 - Stage the record again using the rule for matching
11 - It stages and matches!

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>