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  ± 0      31 suites  ±0   10h 28m 20s ⏱️ - 4m 22s
 4 075 tests + 1   3 969 ✅  -  1    104 💤 ±0  1 ❌ +1  1 🔥 +1 
59 143 runs  +12  56 708 ✅ +10  2 433 💤 ±0  1 ❌ +1  1 🔥 +1 

For more details on these failures and errors, see this check.

Results for commit 3ab7019. ± Comparison against base commit d8fabe2.

♻️ This comment has been updated with latest results.