For each field, application reads separately its value and null-flag.
The null flag is read only when application asks for it.
The default behavior (specified in xsqlda during prepare)
is that null flag is not read for Not Null fields (the rule is a bit
more complex), but there is no problem for application
to ask null flag always.
You can try FSQL tool from this site
http://www.volny.cz/iprenosil/interbase/
it will show null state correctly.

There are other special values that ISQL will not show correctly
(while FSQL will), e.g. try to select minus zero real value:
SELECT -0e0 FROM RDB$DATABASE;

Ivan