Although the CASE WHEN clause returns the same value as the direct concatenation, NULLIF yields different results in each case.

select nullif(SPACE(5), case when false then ('TEST') else ('l' || 'o') end) as c1;

+-------+

| c1    |

+-------+

|       |

+-------+

1 row in set, 2 warnings (0.00 sec)

select nullif(SPACE(5), ('l' || 'o')) as c1;

+------+

| c1   |

+------+

| NULL |

+------+

1 row in set, 3 warnings (0.00 sec)