Way to create unique constraint in Postgres even with null columns

I have a table with this layout:

CREATE TABLE Favorites
(
FavoriteId uuid NOT NULL, --Primary key
UserId uuid NOT NULL,
RecipeId uuid NOT NULL,
MenuId uuid
)

I want to create a unique constraint similar to this:

ALTER TABLE Favorites ADD CONSTRAINT Favorites_UniqueFavorite
UNIQUE(UserId, MenuId, RecipeId);

However, this will allow multiple rows with the same UserId and
RecipeId, if the MenuId is null. I want to allow a NULL MenuId to
store a favorite that has no associated menu, but I only want at most
one of these rows per user/recipe pair.

The ideas I have so far are:

- 1) Use some hard-coded UUID (such as all zeros) instead of null.
However, MenuId has a FK constraint on each user's menus, so I'd then
have to create a special "null" menu for every user which is a
hassle.
- 2) Check for existence of an existing null entry using a trigger
instead. I think this is a hassle and I like avoiding triggers
wherever possible. Plus, I don't trust them to guarantee my data is
never in a bad state.
- 3) Just forget about it and check for the previous existence of a
null entry in the middle-ware or a insert function, and don't have
this constraint.

Is there any method I'm forgetting?

I'm using Postgres 9.0.

Thanks!