Fix variable assignment error with ECS event.start/end

[Technici4n]

Quite straightforward, especially if you look at the surrounding code.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2ca0017) 85.75% compared to head (d106a63) 85.75%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1556   +/-   ##
========================================
  Coverage    85.75%   85.75%           
========================================
  Files          551      551           
  Lines        46334    46334           
========================================
  Hits         39734    39734           
  Misses        6600     6600           

Files Changed	Coverage Δ
.../elastic_ecs/stix_translation/query_constructor.py	95.41% <0.00%> (ø)

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