🐛 [RUMF-71] do not report negative performance timing duration

[BenoitZugmeyer]

No description provided.

[codecov-io]

Codecov Report

Merging #264 into master will increase coverage by 0.82%.
The diff coverage is 88.23%.

@@           Coverage Diff            @@
##           master   #264      +/-   ##
========================================
+ Coverage   85.18%    86%   +0.82%     
========================================
  Files          24     24              
  Lines        1323   1322       -1     
  Branches      276    279       +3     
========================================
+ Hits         1127   1137      +10     
+ Misses        196    185      -11

Impacted Files	Coverage Δ
packages/rum/src/resourceUtils.ts	96.07% <88.23%> (+5.69%)	⬆️
packages/rum/src/rum.entry.ts	71.42% <0%> (+2.04%)	⬆️
packages/core/src/requestCollection.ts	96.15% <0%> (+13.46%)	⬆️

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