Bug 29334: Do not apply defaultvalue to existing auth record

Bug 29334: Do not apply defaultvalue to existing auth record

Note: The check in build_authorized_values_list with defaultvalue can be
simplified since $value may already contain the defaultvalue. It is filled
before the sub is called.
The test for inserting a blank option is generalized; and may be still
improved when using required attributes in the template? Note that we
might experience some browser issues for these placeholders.

Removed a few useless lines with respect to fields controlled
by itemtypes. The last assignment to $value was useless and
its probable intention unwanted (just use defaultvalue).

Test plan:
Put a defaultvalue in an authority framework.
Add a new authority. Check for that value.
Open existing authority. Clear field, save and reopen. Check if
field is still empty.
Bonus: test field controlled by authorised values.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>