"reverse" (?) UPSERT -- how to ?

Dear list members,

maybe I am overlooking something.

PostgreSQL offers UPSERT functionality by way of

    INSERT INTO ... ON CONFLICT ... DO UPDATE ...;

Consider this pseudo-code schema

    table master
        pk_master serial primary key
        value text
    ;

    table detail
        pk_detail serial primary key
        fk_master int foreign key master(pk_master)
        detail text
    ;

where there can be any number of rows in detail linking to a
particular row in master, including none (master rows without
details, that is).

There will be a view giving rows for
    each detail row enriched with master table data
        UNION ALL
    rows for each master row that does not have any detail row with detail table columns NULLed

What I want to achieve:

Given a pk_detail (and pk_master) having been obtained from
the view (therefore pk_detail being NULL or an integer value)
UPDATE that detail row (namely when pk_detail is distinct
from NULL) or insert a new detail row (when pk_detail IS
NULL) linking that row to the master row identified by
pk_master.

I know I can do so from client code. I also know I can wrap
this functionality inside a plpgsql function.

I am wondering though whether it can be done as one SQL
statement. It seems to me that would call for an

    UPDATE ... ON MISSING ... DO INSERT ...;

or

    CASE
        WHEN pk_detail IS NULL THEN INSERT ...
        WHEN pk_detail IS DISTINCT FROM NULL THEN UPDATE ...
    END

both of which don't exist/work, of course.

So, is this doable within one SQL statement (short of
creating and running the abovementioned function in
one go ;-) ?

(the real story is about medication and intake regimens /
schedules thereof involving a whole lot more columns, of
course, which should not be relevant to the problem though)

Thanks for taking the time to read,
Karsten
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