scrypted: log attachListener early-return when no camera assigned

Silent early-return at 'if (!v.cameraId) return;' makes a brand-new
viewport with no camera selected look identical (in the console) to
one that subscribed successfully — there's no positive or negative
signal until you try to fire a camera event. After observing a fresh
viewport produce zero output on a doorbell press, switching the
early-return to a warning that says 'no camera assigned — open
Settings and pick a camera; subscription skipped' so the missing
configuration becomes self-evident.

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