Oracle function based index with empty function name
That is not an unnamed/empty function. That is a column expression made from a constant, between parentheses.

And it is listed as function-based index, because: 

CREATE INDEX


  When you specify column_expression, you create a function-based index.


The below is a function-based index too by definition, while it is nothing more than the a constant for all rows.

CREATE INDEX IDX3 ON t('a');