SliceInfo: Defining container_serialized as empty, before trying to serialize
SliceInfo: Defining container_serialized as empty, before trying to serialize

While discussing with Jordy about the code I just found a problematic case where sending slice info could fail.
(This shouldn't happen, but..) If the first container is not serializable, then the for-loop will crash because container_serialized is not set for the if clause.
Additionally the if-clause is at the moment useless, because it is always true, because the content of container_serialized gets never resetted.

Contributes to CURA-1445
Curahelper/Cura,ynotstartups/Wanhao,hmflash/Cura,fieldOfView/Cura,fieldOfView/Cura,Curahelper/Cura,hmflash/Cura,ynotstartups/Wanhao