Fix handling of empty documentation header

[blink1073]

Fixes #226

[codecov-commenter]

Codecov Report

Merging #230 (4d730dd) into master (f827a42) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
- Coverage   85.14%   85.13%   -0.02%     
==========================================
  Files          19       19              
  Lines        2309     2307       -2     
  Branches      291      291              
==========================================
- Hits         1966     1964       -2     
  Misses        247      247              
  Partials       96       96              

Impacted Files	Coverage Δ
jupyter_releaser/changelog.py	87.03% <0.00%> (-0.24%)	⬇️
jupyter_releaser/tests/util.py	99.21% <ø> (ø)

---

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 f827a42...4d730dd. Read the comment docs.