Check sectionsJSON was not null before trying to access .length() in SectionsFetchTask.processResult

https://ticket.wikimedia.org/otrs/index.pl?Action=AgentTicketZoom;TicketID=7708614
https://ticket.wikimedia.org/otrs/index.pl?Action=AgentTicketZoom;TicketID=7735170

Change-Id: If4667ea5e068c28b8daf21769dedcdb079fe932c
Merge "Check sectionsJSON was not null before trying to access .length() in SectionsFetchTask.processResult"