editoast: study with missing dates are valid

[Khoyo]

Fixes #5560.

[codecov]

Codecov Report

Merging #5637 (db58b6d) into dev (e5075d3) will increase coverage by 0.00%.
The diff coverage is 88.09%.

@@            Coverage Diff            @@
##                dev    #5637   +/-   ##
=========================================
  Coverage     19.61%   19.62%           
  Complexity     2322     2322           
=========================================
  Files           880      880           
  Lines        105594   105610   +16     
  Branches       2566     2566           
=========================================
+ Hits          20715    20727   +12     
- Misses        83376    83380    +4     
  Partials       1503     1503           

Flag	Coverage Δ
editoast	67.95% <88.09%> (+0.01%)	⬆️

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Files	Coverage Δ
editoast/src/models/projects.rs	66.21% <90.90%> (+1.93%)	⬆️
editoast/src/models/study.rs	90.32% <81.81%> (-4.92%)	⬇️
editoast/src/views/study.rs	94.18% <90.00%> (+2.51%)	⬆️

... and 3 files with indirect coverage changes