Fix nil start_timestamp or nil end_timestamp

[Rados13]

No description provided.

[codecov]

Codecov Report

Merging #389 (b74e92e) into master (b2791b0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
+ Coverage   54.17%   54.18%   +0.01%     
==========================================
  Files          75       75              
  Lines        3258     3259       +1     
==========================================
+ Hits         1765     1766       +1     
  Misses       1493     1493              

Files	Coverage Δ
recording/lib/reporter.ex	100.00% <100.00%> (ø)

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