Why does adding count(*) to a select statement force a row to exist in a subquery?

Question

On Oracle 9i, why does the following produce the result 'abc'

select 'abc ' || (select txt from 
     (select 'xyz' as txt from dual where 1=2)) 
from dual

while this produces 'abc xyz':

select 'abc ' || (select txt from 
     (select count(*), 'xyz' as txt from dual where 1=2)) 
from dual

Why does adding count(*) to the subquery result in different behavior? Should the predicate where 1=2 preclude any results in the subquery?

Answer 1

select count(*) from dual where 1=2

returns 0. That is, a row with the value zero.