Protect against unset spec

[fcollonval]

Fixes #555

[codecov-commenter]

Codecov Report

Merging #556 (269e3c5) into master (b4b1517) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
- Coverage   76.98%   76.90%   -0.08%     
==========================================
  Files         109      109              
  Lines        9945     9948       +3     
  Branches     1078     1078              
==========================================
- Hits         7656     7651       -5     
- Misses       1909     1914       +5     
- Partials      380      383       +3     

Impacted Files	Coverage Δ
jupyter_server/tests/test_utils.py	100.00% <100.00%> (ø)
jupyter_server/utils.py	65.06% <100.00%> (+0.64%)	⬆️
jupyter_server/services/kernels/handlers.py	61.37% <0.00%> (-1.59%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	80.14% <0.00%> (-0.73%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4b1517...269e3c5. Read the comment docs.

[blink1073]

Thank you!

[blink1073]

Cutting a patch release now