fix: don't update missing notification

[Xarthisius]

Only update notification in updateNotification if it exists. Fixes #559

[codecov[bot]]

Codecov Report

Base: 92.81% // Head: 92.81% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (858b9f5) compared to base (fc1322e). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #560 +/- ## ========================================== - Coverage 92.81% 92.81% -0.01% ========================================== Files 60 60 Lines 4787 4786 -1 ========================================== - Hits 4443 4442 -1 Misses 344 344 ``` | [Impacted Files](https://codecov.io/gh/whole-tale/girder_wholetale/pull/560?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=whole-tale) | Coverage Δ | | |---|---|---| | [server/\_\_init\_\_.py](https://codecov.io/gh/whole-tale/girder_wholetale/pull/560/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=whole-tale#diff-c2VydmVyL19faW5pdF9fLnB5) | `90.59% <100.00%> (-0.03%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=whole-tale). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=whole-tale)

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