How to create an "empty" partial index, or equivalent, in Postgres?
You are saying "rows" where I think you mean "columns".

It is not legal to specify an empty list of columns in an index.  Just pick a column, preferably one with a short data type.  If nothing else, just repeat the same column in both places.

CREATE INDEX mytable_cond ON mytable (cond) WHERE cond = TRUE;


Be careful how you formulate our index and queries.  A cond = true query cannot use a partial index defined where cond is true and vice versa, those are not considered equivalent operations.