ref: use non-nullable .get() lookup instead of .first()

[asottile-sentry]

this would always crash on line 263 if it were nullable

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.10%. Comparing base (84e405e) to head (181190a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #75115   +/-   ##
=======================================
  Coverage   78.10%   78.10%           
=======================================
  Files        6755     6755           
  Lines      301462   301462           
  Branches    51861    51861           
=======================================
+ Hits       235445   235453    +8     
+ Misses      59686    59679    -7     
+ Partials     6331     6330    -1     

Files	Coverage Δ
.../sentry/api/endpoints/organization_events_trace.py	85.56% <100.00%> (ø)

... and 3 files with indirect coverage changes