Bug 17389: Clear logs

Bug 17389: Clear logs

Logs are full of:
Use of uninitialized value in substitution (s///) at
C4/ImportExportFramework.pm line 282.
Use of uninitialized value in concatenation (.) or string at
C4/ImportExportFramework.pm line 283.

Some field are NULL in DB, we need to init the value to an empty string
to avoid these warnings.

Note: This fix is not related to the original bug
Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Warnings avoided

Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit a1dcd2f2e67f2dc998d2eae0fc6097eb9edbb874)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit 4592382e388d7f0ee0467180b50b5630a19c674a)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>