🐛 Fixed returning 'null' text for exerpt helper
🐛 Fixed returning 'null' text for exerpt helper

refs #10554

- Fixes a flow described in https://github.com/TryGhost/Ghost/issues/10554#issuecomment-469127056

Gargol/Ghost,TryGhost/Ghost,kevinansfield/Ghost,ErisDS/Ghost,Gargol/Ghost,Kikobeats/Ghost,JohnONolan/Ghost,TryGhost/Ghost,e10/Ghost,ErisDS/Ghost,bitjson/Ghost,janvt/Ghost,Kikobeats/Ghost,e10/Ghost,ErisDS/Ghost,JohnONolan/Ghost,bitjson/Ghost,kevinansfield/Ghost,JohnONolan/Ghost,janvt/Ghost,Kikobeats/Ghost,kevinansfield/Ghost,TryGhost/Ghost