Fix SamplePath for constant data

[dlindhol]

We use SamplePath (List[SamplePosition]) to identify the location of a data value within a Sample. (Nested Functions add to the list.)

We've been able to support the idea of a ConstantFunction that allows us to have a Dataset with a single constant data value in a 0-arity Sample (empty list of domain values). DataType.getPath currently returns an empty SamplePath for a single Scalar and breaks with a MatchError for Tuple. These should be able to point to the appropriate RangePosition of the 0-arity Sample.