Handle case where $input['order'] is set but empty.
Handle case where $input['order'] is set but empty.

Re #628 fix issue when saving a modular child folder as 05.something
and being reset to 01.something upon save

getgrav/grav-plugin-admin,Sommerregen/grav-plugin-admin,getgrav/grav-plugin-admin,dimayakovlev/grav-plugin-admin,dimayakovlev/grav-plugin-admin,Sommerregen/grav-plugin-admin,Sommerregen/grav-plugin-admin,dimayakovlev/grav-plugin-admin,dimayakovlev/grav-plugin-admin,getgrav/grav-plugin-admin,Sommerregen/grav-plugin-admin