Use TREAT to restrict subtypes in a query in Oracle

Question

According to the Oracle docs, i can use TREAT to return a specific subtype in a query, see Example 2-38

However, the docs also state that for any subtypes that aren't students (in the example), it returns NULL.

Since i am using PUT_LINE to output a report, with headings "STUDENT NAME: " etc, it is outputting rows of empty headings.

How can i stop this behaviour?

Thanks