[mcs] Fix compiling "string"+object if corlib don't have nullable types

With commit 09ca788b4b0c7c1fc3e84cf96dda1440001a79c3 when string+(object, int, double...) addition operator was moved into lifted group with nullables in case nullables are not defined in corlib(e.g. MicroFramework corlib doesn't have nullables) mcs didn't compile this statment: string newStringValue="someString"+1; but instead generate error: CS0019: Operator '+' cannot be applied to operands of type 'string' and 'int'