Type annotations for deque should include `| null`

[jdunck]

These methods could return null:

• popFront
• popBack
• front
• back

The docs seem to indicate they do but the type annotations do not.

I think we should either add | null here, or we should raise when the deque is empty. I'd actually prefer that, since right now, Deque can't store a null-value.

What is your preference?

[jdunck]

@eyas-ranjous happy to do this, but do you have a preference whether to annotate just with existing behavior, or would you like to support telling the difference between empty deque and deque w null value?

[eyas-ranjous]

@jdunck hi. just adding null to the return of the functions would be good. thanks