Bug 12874: A DB field without a default mapping is set to a default value on editing

If an item is edited and a field is not mapped to Koha, it is to 0 or
NULL (depending on the default value defined).

This patch adds a check on the mapping before editing the item. It there
is no mapping, the DB value is not erased.

Test plan:
1/ Edit an item and fill a value for a field
2/ Unmap this field
3/ Edit the item
4/ Verify that the value is not erased (using the MySQL CLI)

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>