asyncio: Fix setting AsyncTimeout._on_timeout trigger, when timeout=0
asyncio: Fix setting AsyncTimeout._on_timeout trigger, when timeout=0

## Bug description
I wanted to initialise `AsyncTimeout` state with timeout=0, so I can set timeout after initialisation before/after entering state. But trigger is not called after setting timeout > 0, entering the state and timeout has passed.

So in my project, I set on_timeout along with the timeout before entering to state and it's working.

## Root cause analyse result
`AsyncTimeout._on_timeout` is a list of triggers and there is a `on_timeout` setter that listifies value before setting. `AsyncTimeout.__init__` uses setter if `timeout > 0`, but it passes directly if `timeout = 0`.

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