In a partition why 1st time encounter NULL then call minvfunc

same question:
https://stackoverflow.com/questions/73476732/postgresql-in-a-partition-1st-time-null-then-call-minvfunc
dbfilddle
<https://dbfiddle.uk/?rdbms=postgres_14&fiddle=307e1fa8d83af57aeb7698d9d58056cf>

source
<https://git.postgresql.org/gitweb/?p=postgresql.git;f=src/test/regress/expected/window.out;hb=5c292e6b90433c760a3e15027646c7b94afd0cdd#l3319>

create or replace function logging_msfunc_strict(text,anyelement)returns text as
$$select $1 || '+' || quote_nullable($2)
$$LANGUAGE sql strict IMMUTABLE;
create or replace function logging_minvfunc_strict(text,
anyelement)returns text as
$$select $1 || '-' || quote_nullable($2)
$$LANGUAGE sql strict IMMUTABLE;
create aggregate logging_agg_strict(text)
(
stype = text,
sfunc = logging_sfunc_strict,
mstype = text,
msfunc = logging_msfunc_strict,
minvfunc = logging_minvfunc_strict
);

create aggregate logging_agg_strict_initcond(anyelement)
(
stype = text,
sfunc = logging_sfunc_strict,
mstype = text,
msfunc = logging_msfunc_strict,
minvfunc = logging_minvfunc_strict,
initcond = 'I',
minitcond = 'MI'
);

execute following query:

SELECT
p::text || ',' || i::text || ':' || COALESCE(v::text, 'NULL') AS _row,
logging_agg_strict (v) OVER w AS nstrict,
logging_agg_strict_initcond (v) OVER w AS nstrictFROM (
VALUES (1, 1, NULL),
(1, 2, 'a'),
(1, 3, 'b'),
(1, 4, NULL),
(1, 5, NULL),
(1, 6, 'c'),
(2, 1, NULL),
(2, 2, 'x'),
(3, 1, 'z')) AS t (p, i, v)
WINDOW w AS (PARTITION BY p ORDER BY i ROWS BETWEEN 1 PRECEDING
AND CURRENT ROW);

return following result:

_row | nstrict | nstrict----------+-----------+----------------
1,1:NULL | [[null]] | MI
1,2:a | a | MI+'a'
1,3:b | a+'b' | MI+'a'+'b'
1,4:NULL | a+'b'-'a' | MI+'a'+'b'-'a'
1,5:NULL | [[null]] | MI
1,6:c | c | MI+'c'
2,1:NULL | [[null]] | MI
2,2:x | x | MI+'x'
3,1:z | z | MI+'z'
(9 rows)

For now I don't understand row 1,4:NULL | a+'b'-'a' | MI+'a'+'b'-'a'. I am
not sure why the 1st time you encounter NULL then it will call inverse
transition function Overall, not sure about the idea of inverse transition
function.