event: make nil available

[cuiweixie]

Description
Problem

When FeedOf is instantiated with an interface-shaped type argument (notably any / interface{}) and callers use Send(nil), the logical value is nil carried inside an interface box. Taking reflect.ValueOf(value) in that situation yields an invalid reflect.Value, because the boxed value does not expose a concrete type the way reflect expects.

That invalidated value was then plugged into reflective send helpers (reflect.SelectCase, reflect.Value.TrySend). The old behaviour was wrong in practice: Send(nil) could panic or fail to deliver, instead of broadcasting nil to subscribers like any other payload.

Fix

Build the reflect.Value used for sending from reflect.ValueOf(&value).Elem(), so the holder of T is reflected correctly and the send path sees a valid value (including “nil interface” cases).

Testing

Add TestFeedOfSendNilAnySucceeds, which subscribes two buffered channels on a FeedOf[any], calls Send(nil), and asserts nsent == 2 and that both subscribers receive nil. This locks in the fixed behaviour and documents the previous failure mode.

[fjl]

Please give justification for this change!

[cuiweixie]

Please give justification for this change!

Done