Handle empty MultiProgressWidget shutdown

[peter941221]

Closes #3007

MultiProgressWidget only initialized �lapsed_time inside make_widget(). When progress tracking finished without ever creating any bars, _draw_stop() could still run and access self.elapsed_time, raising AttributeError.

This change initializes �lapsed_time during widget construction so shutdown paths stay safe even when there are no task groups to render.

It also adds a focused regression test covering the empty-keys shutdown path.

Validation:

• python -m py_compile distributed/diagnostics/progressbar.py distributed/diagnostics/tests/test_progress_widgets.py
• python -m black --check distributed/diagnostics/progressbar.py distributed/diagnostics/tests/test_progress_widgets.py
• Local pytest could not run in this environment because dask is not installed and pytest fails during initial config import

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    31 files  +    1      31 suites  +1   10h 36m 51s ⏱️ +26s
 4 081 tests +    1   3 969 ✅ +    5    112 💤  -  1  0 ❌  - 2 
59 230 runs  +1 410  56 628 ✅ +1 347  2 602 💤 +66  0 ❌  - 2 

Results for commit 28cd49f. ± Comparison against base commit c9ef602.

♻️ This comment has been updated with latest results.

[peter941221]

Rebased on current main and pushed 28cd49f. CI is running again.