MOBILE-786 messages: Do not display empty messages
MOBILE-786 messages: Do not display empty messages

This may happen in older installations due to MDL-44954

taher552/tahfiz,danielsonnora/universidaderaffinato,navdeep300/moodlemobile,bakasa/moodle101,Namikase/moodlemobile,ingeniuslearning/moodlemobile,Vedran91/moodlemobile,ingoscarmorenoc/ios2,ingoscarmorenoc/ios7,bakasa/moodle101,moodlewebexapp/moodlemobile,Nithis111/AIT-Tech,4r1y4n/moodlemobile,clopezlga/moodlemobile,moodlehq/moodlemobile,ingoscarmorenoc/ios3,oustaban/moodlemobile,vincent-nephila/amaumobile,akinibrahim/moodlemobile,vincent-nephila/amaumobile,dianjiaotong/dianjiaotong_apps_android,ingoscarmorenoc/ios6,ingoscarmorenoc/ios5,ingoscarmorenoc/ios4,danielsonnora/universidaderaffinato,ardock/moodlemobile,dianjiaotong/moodle_apps_android,dianjiaotong/dianjiaotong_apps_android,vmoll/moodlemobile,tang7526/moodlemobile,ubaldojmp/Moodle-CUC,muhammadumarpatel/felarc,iharris78/ctcswlapp,GG001/GG,CEDITE/FACETVirtual.Mobile-OLD,oustaban/moodlemobile,elvincasem/moodlemobileous,CEDITE/FACETVirtual.Mobile-OLD,4r1y4n/moodlemobile,navdeep300/moodlemobile,xrdcode/moodlemobile,Vedran91/moodlemobile,moodlehq/moodlemobile,ingoscarmorenoc/ios3,taher552/tahfiz,techguru123/MoodM,elvincasem/moodlemobileous,vmoll/moodlemobile,GG001/GG,xrdcode/moodlemobile,ingeniuslearning/moodlemobile,tang7526/moodlemobile,guozhenqiang/hit,Ilmawankh/modle-elearning,moodlewebexapp/moodlemobile,Nithis111/AIT-Tech,sudeepnayak/moodlemobile,yusa55/moodlemobile,ingoscarmorenoc/LLLLL,ubaldojmp/Moodle-CUC,ingoscarmorenoc/ios2,Ilmawankh/modle-elearning,sergiopro14/moodlemobile,ingoscarmorenoc/ios5,akinibrahim/moodlemobile,sergiopro14/moodlemobile,dianjiaotong/moodle_apps_android,ardock/moodlemobile,guozhenqiang/hit,sudeepnayak/moodlemobile,clopezlga/moodlemobile,ingoscarmorenoc/ios6,Namikase/moodlemobile,stefobark/moodlemobile,stefobark/moodlemobile,techguru123/MoodM,ingoscarmorenoc/ios4,iharris78/ctcswlapp,ingoscarmorenoc/ios7,muhammadumarpatel/felarc