Fix AwaitMessageTrigger crash on tombstone messages when apply_function is unset

[TimurRakhmatullin86]

When AwaitMessageTrigger is used without an apply_function (supported since #55437), the trigger calls message.value().decode("utf-8") on every polled message. For a tombstone message — value() is None, a routine occurrence on log-compacted topics where a null value is the deletion marker — this raises AttributeError: 'NoneType' object has no attribute 'decode', crashing the trigger and failing the deferred task.

This PR treats a tombstone the same way as any other non-matching message: no event is emitted, the offset is committed (when commit_offset=True, matching the existing non-matching path so the tombstone is not re-read forever after a restart), and the trigger keeps polling.

Why skip rather than emit: the trigger's contract is already truthiness-based (if event:), so a None payload cannot be yielded on this path today, and a TriggerEvent(None) would carry no usable payload downstream. Users who need to react to tombstones can already do so via apply_function, which receives the raw message (including value() is None) and can map it to any truthy payload. Surfacing tombstones from the no-apply_function convenience path would be a separate opt-in feature; crashing on them is a bug.

Tests:

• test_trigger_run_tombstone_message_keeps_polling — regression test: poll() returns a null-value message; asserts the trigger neither raises nor yields and commits the tombstone offset. Fails on main with the exact AttributeError above.
• test_trigger_run_without_apply_function_yields_message_value — asserts the happy path still yields the decoded payload.
• MockedMessage now has a value() method: the apply_function=None case of test_trigger_run_good previously passed for the wrong reason — the mock lacked value(), so the task finished via this same AttributeError, satisfying task.done() is True.

No behavior change for non-null messages or when apply_function is set. Empty (b"") payloads were already treated as non-events by the if event: check — unchanged.