nova:subscribe - fix exception in category's callback when a post isn't assigned to a category
nova:subscribe - fix exception in category's callback when a post isn't assigned to a category

nathanmelenbrink/theRecord,ahmadassaf/Telescope,Discordius/Telescope,basemm911/questionsociety,dominictracey/Telescope,acidsound/Telescope,bshenk/projectIterate,TodoTemplates/Telescope,SachaG/Gamba,TodoTemplates/Telescope,bshenk/projectIterate,ahmadassaf/Telescope,SachaG/Zensroom,rtluu/immersive,SachaG/Gamba,julienlapointe/telescope-nova-social-news,Discordius/Lesswrong2,jonmc12/heypsycho,Discordius/Lesswrong2,SachaG/Gamba,nathanmelenbrink/theRecord,nathanmelenbrink/theRecord,VulcanJS/Vulcan,bengott/Telescope,SachaG/Zensroom,bshenk/projectIterate,HelloMeets/HelloMakers,Air2air/Telescope,SachaG/Zensroom,acidsound/Telescope,bengott/Telescope,jonmc12/heypsycho,acidsound/Telescope,dominictracey/Telescope,Discordius/Telescope,Air2air/Telescope,TodoTemplates/Telescope,manriquef/Vulcan,manriquef/Vulcan,basemm911/questionsociety,VulcanJS/Vulcan,basemm911/questionsociety,HelloMeets/HelloMakers,Discordius/Lesswrong2,JstnEdr/Telescope,HelloMeets/HelloMakers,bengott/Telescope,jonmc12/heypsycho,JstnEdr/Telescope,rtluu/immersive,ahmadassaf/Telescope,Discordius/Lesswrong2,Discordius/Telescope,Discordius/Telescope,dominictracey/Telescope,Air2air/Telescope,manriquef/Vulcan,JstnEdr/Telescope,rtluu/immersive,julienlapointe/telescope-nova-social-news,julienlapointe/telescope-nova-social-news