Can an Oracle IDENTITY column be nullable

I want to create an ID column that automatically generates unique ID numbers by default.

Users won't be inserting rows via INSERT statements, rather, they'll create rows using an out-of-box application (using the attribute editing environment).

I've heard that auto-generated IDs can be achieved by creating an Oracle IDENTITY column
(hint: to find the relevant section in the linked page, search for this text: Use this clause to specify an identity column.).

And so, I have successfully created a table with an IDENTITY column:

CREATE TABLE A_TEST_TABLE (ID NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY);

However, I see that the field is not nullabe:

SELECT ID FROM ALL_TAB_COLUMNS WHERE TABLE_NAME = 'A_TEST_TABLE'

NULLABLE = N

This is a problem because my application tries to "do users a favor". If a field is not nullable, the application automatically inserts a zero as a dummy value. This overwrites the auto-generated ID (which of course, is not what I want). I'm guessing that it does this to avoid getting an error message from the database if a null value is committed.

To avoid this issue with my application, is it possible to make the field nullabe?

When I read the documentation, it says:

If you specify ON NULL, then Oracle Database uses the sequence
generator to assign a value to the column when a subsequent INSERT
statement attempts to assign a value that evaluates to NULL.

Does this imply that an IDENTITY field can be nullable? If so, how?

I recognize that I have no idea what I'm doing. I don't even think I would ultimately want an ID field that is nullable. I'm just trying to make sense of all this, and I'm failing.