Handle Nullable Value Types

[bruce-dunwiddie]

There's probably a missing nullable type check along with correct cast handling in the Linq Expression tree.

var evaluator = new Evaluator(
    "return someNumber");

evaluator.SetVariable(
    "someNumber",
    null,
    typeof(decimal?));

Assert.IsNull(evaluator.Eval<decimal?>());

System.InvalidCastException : Null object cannot be converted to a value type.

[bruce-dunwiddie]

This passes however:

var evaluator = new Evaluator(
    "return someNumber");

evaluator.SetVariable(
    "someNumber",
    null,
    typeof(decimal?));

Assert.IsNull(evaluator.Eval());