Skip no-op task if possible

[pencil]

This PR optimizes the handling of empty Chain or Group workflows. Instead of scheduling a noop task and relying on the middleware to process completion callbacks, this change processes them directly within the Workflow.run method when there is no delay. This avoids an unnecessary round trip to the broker.

Why do we even support empty Chains and Groups?

Empty chains and groups are common when created programmatically:

Chain(
  Group(*[do_a_bunch_of_work.message(id) for id in work_items]),
  finalize_work.message(),
)

In this example, work_items might be empty, in which case finalize_work should run immediately.

[Copilot]

Pull Request Overview

Optimizes empty workflow handling by processing completion callbacks without scheduling an unnecessary no-op task when no delay is specified.

• Adds tests for empty Chain and Group workflows.
• Implements direct callback processing in Workflow.__schedule_noop when no delay is provided.
• Introduces a new _process_completion_callbacks method in the middleware.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dramatiq_workflow/tests/test_workflow.py	Adds tests ensuring that empty workflows skip broker enqueue and process callbacks correctly.
dramatiq_workflow/_middleware.py	Introduces _process_completion_callbacks to process workflow completion callbacks immediately.
dramatiq_workflow/_base.py	Updates __schedule_noop to call _process_completion_callbacks when there's no delay, avoiding a no‑op task.

[dan-woz]

coolio 😎