Fix crash on missing env var in dashboard link formatting

[milesgranger]

Closes #7724

• Tests added / passed
• Passes pre-commit run --all-files

[jacobtomlinson]

Looks good! Thanks for jumping on this so quickly.

[github-actions]

Unit Test Results

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

       26 files  +       24         26 suites  +24   14h 37m 46s ⏱️ + 13h 46m 53s
  3 549 tests +       52    3 437 ✔️ +       52     106 💤  -        6  6 ❌ +6 
44 897 runs  +41 389  42 650 ✔️ +39 265  2 240 💤 +2 117  7 ❌ +7 

For more details on these failures, see this check.

Results for commit 491fef6. ± Comparison against base commit e1944ec.

This pull request removes 1 and adds 53 tests. Note that renamed tests count towards both.

distributed.tests.test_scheduler ‑ test_multiple_listeners

distributed.cli.tests.test_dask_scheduler
distributed.cli.tests.test_dask_scheduler ‑ test_signal_handling[15]
distributed.cli.tests.test_dask_scheduler ‑ test_signal_handling[2]
distributed.cli.tests.test_dask_ssh
distributed.cli.tests.test_dask_worker ‑ test_signal_handling[15---nanny]
distributed.cli.tests.test_dask_worker ‑ test_signal_handling[15---no-nanny]
distributed.cli.tests.test_dask_worker ‑ test_signal_handling[2---nanny]
distributed.cli.tests.test_dask_worker ‑ test_signal_handling[2---no-nanny]
distributed.dashboard.tests.test_components
distributed.dashboard.tests.test_scheduler_bokeh
…

♻️ This comment has been updated with latest results.

[jacobtomlinson]

Awesome thanks @milesgranger. Looks like black is unhappy now though.

[milesgranger]

Ready for a review @jrbourbeau @j-bennet @hendrikmakait
Thanks in advance. :)

[hendrikmakait]

Thanks, @milesgranger! CI failures appear to be unrelated.