Bug 3727: Corrects the behaviour of the itemtype selectbox in addbiblio.pl

Bug 3727: Corrects the behaviour of the itemtype selectbox in addbiblio.pl

If the subfield is set as mandatory with an empty default value, an
empty value is added in the selectbox and this value is selected by
default. Before, the first element was automatically selected, so it was
easy to forget to set this mandatory value.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
(cherry picked from commit f7d9ebbff3ab9b620b0c0b80b3dc28c51753ec13)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>