Add IS_EMPTY Function.

When analyzing data, there is currently no single function to evaluate whether a given field is empty.  With scalar fields, this can be accomplished with the `IS NOT NULL` operator, but with complex fields, this is more challenging as complex fields are never null. 

This PR adds a UDF called IS_EMPTY() which accepts any type of field and returns true if the field does not contain data.  

In the case of scalar fields, if the field is `null` this returns true.  In the case of complex fields, which can never be `null`, in the case of lists, the function returns true if the list is empty.  In the case of maps, it returns true if all of the map's fields are unpopulated.