default fallback url should be unique on update
default fallback url should be unique on update

Fallback url should be unique on every update, not only on creation of new store.

Fixes #6010

grzlus/solidus,grzlus/solidus,lsirivong/solidus,lsirivong/solidus,Senjai/solidus,forkata/solidus,athal7/solidus,bonobos/solidus,Senjai/solidus,athal7/solidus,net2b/spree,bonobos/solidus,scottcrawford03/solidus,devilcoders/solidus,ckk-scratch/solidus,pervino/solidus,Senjai/solidus,jordan-brough/solidus,jsurdilla/solidus,pervino/solidus,xuewenfei/solidus,athal7/solidus,jsurdilla/solidus,jordan-brough/solidus,Arpsara/solidus,xuewenfei/solidus,richardnuno/solidus,grzlus/solidus,richardnuno/solidus,Arpsara/solidus,athal7/solidus,forkata/solidus,forkata/solidus,Senjai/solidus,net2b/spree,ckk-scratch/solidus,grzlus/solidus,bonobos/solidus,jordan-brough/solidus,jordan-brough/solidus,pervino/solidus,richardnuno/solidus,bonobos/solidus,jsurdilla/solidus,devilcoders/solidus,richardnuno/solidus,lsirivong/solidus,jsurdilla/solidus,ckk-scratch/solidus,scottcrawford03/solidus,net2b/spree,lsirivong/solidus,Arpsara/solidus,ckk-scratch/solidus,forkata/solidus,Arpsara/solidus,xuewenfei/solidus,xuewenfei/solidus,scottcrawford03/solidus,scottcrawford03/solidus,devilcoders/solidus,devilcoders/solidus,net2b/spree,pervino/solidus