An representation of a nullable data value.
Tokens in the DataRush framework support a concept of null value
analogous to SQL. Specifically, a token may not have a value assigned,
in which case it is referred to as null valued.
Tokens which do have a value are called non-null and contain
a value appropriate to their type.
Indicates whether the token is null valued. Tokens support null
values analogous to SQL. Before accessing the value of the container,
it is usually best to ensure it is not null valued using this method.