For a question about NULL to make any sense, the context and/or the operational definition must be set.

NULL has different results in the same operations based on other settings, therefore, NULL has different definitions based on those settings.

None of that is counting certain segments of actual practice, (not theory) in which the operational definition of NULL is actually 'Treat it as you treat empty string or 0 or some other known value, because those who came before failed to set the column to be NOT NULL for one or more reasons, and we aren't allowed to change the data or the setting now'.