Test JavaScriptSerializer.Deserialize with nullable enums. Covers #19287.
Improved JavaScriptSerializer.ConvertToType Nullable handling. Fixes #19287. ConvertToType was returning null even when the Nullable object was not a string. When the target type is Nullable, ConvertToType returns null if the object is an empty string, if the object is not a string and the target type is obtainable from a string then the object is converted to a string and the string to the target type.