Fix crash in chart api when some sample values are null (fixes #365)

[aptiko]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[codecov[bot]]

Codecov Report

Merging #374 (1bada52) into master (8799246) will increase coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   96.99%   97.13%   +0.14%     
==========================================
  Files          20       20              
  Lines        1399     1398       -1     
==========================================
+ Hits         1357     1358       +1     
+ Misses         42       40       -2     

Impacted Files	Coverage Δ
enhydris/api/serializers.py	95.23% <100.00%> (ø)
enhydris/api/views.py	97.26% <100.00%> (+0.89%)	:arrow_up:

---

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 8799246...1bada52. Read the comment docs.