Empty sum problem

[msakai]

It is possible to generate OPB files that contain empty <sum>.

> putStr $ toOPBString $ Formula{ pbObjectiveFunction = Just [], pbConstraints = [], pbNumVars = 0, pbNumConstraints = 0 }
* #variable= 0 #constraint= 0
min: ;

> putStr $ toOPBString $ Formula{ pbObjectiveFunction = Nothing, pbConstraints = [([], Ge, 0)], pbNumVars = 0, pbNumConstraints = 1 }
* #variable= 0 #constraint= 1
>= 0;

But empty <sum> is not allowed in the grammar:

<sum>::= <weightedterm> | <weightedterm> <sum>

and our parser based on this grammar rejects it.

This causes parsing errors.
e.g. msakai/toysolver#60 (comment)