[#77959454] nullify database empty strings and [#77538404] set items.owner_id as not null

Squashed commit of the following:

commit 63bb7223f32f9b16e0491873bdea79ec6fa335ba
Author: Franco Sellitto <franco.sellitto@zhdk.ch>
Date:   Tue Sep 2 12:09:51 2014 +0200

    [#75876230] button translation

commit 5ef69d420534a6c1a27b7dbb0521a15ea36aafe6
Author: Franco Sellitto <franco.sellitto@zhdk.ch>
Date:   Tue Sep 2 11:46:14 2014 +0200

    item#owner_id is now also required

commit 39980022315baf22a6ee932a9e0c3bc33d08d409
Author: Franco Sellitto <franco.sellitto@zhdk.ch>
Date:   Mon Sep 1 17:09:03 2014 +0200

    migration and admin check for empty strings