Test cases for predicates distributed into the union sub-parts:

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = ''

PLAN (DT RDB$RELATIONS INDEX (RDB$INDEX_0))
PLAN (DT RDB$PROCEDURES INDEX (RDB$INDEX_21))

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = upper('')

PLAN (DT RDB$RELATIONS INDEX (RDB$INDEX_0))
PLAN (DT RDB$PROCEDURES INDEX (RDB$INDEX_21))

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = trim('')

PLAN (DT RDB$RELATIONS INDEX (RDB$INDEX_0))
PLAN (DT RDB$PROCEDURES INDEX (RDB$INDEX_21))

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = left('', 0)

PLAN (DT RDB$RELATIONS NATURAL)
PLAN (DT RDB$PROCEDURES NATURAL)

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = minvalue('', '')

PLAN (DT RDB$RELATIONS NATURAL)
PLAN (DT RDB$PROCEDURES NATURAL)

select * from (
  select rdb$relation_name from rdb$relations
  union
  select rdb$procedure_name from rdb$procedures
) as dt (name) where dt.name = rpad('', 0, '')

PLAN (DT RDB$RELATIONS NATURAL)
PLAN (DT RDB$PROCEDURES NATURAL)

The issue does not affect v3.0.