🐛 [RUMF-931] check if PerformanceEntry is defined before using it

[BenoitZugmeyer]

Motivation

When implementing #883, we needed to use the global PerformanceEntry class. This class is not supported in Safari 10 and below.
Fix #890

Changes

Check if PerformanceEntry is defined before using it.

Testing

CI, manual (browser stack live with Safari 10)

Note

This should have been an internal error, but because of the await used in the fetchProxy, it ends up as an "unhandled rejection" (anything executed after the await is not "monitored"). We should address this issue in a separate PR.

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #891 (2329f80) into main (a846216) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
- Coverage   89.14%   89.13%   -0.02%     
==========================================
  Files          81       81              
  Lines        3804     3809       +5     
  Branches      850      850              
==========================================
+ Hits         3391     3395       +4     
- Misses        413      414       +1     

Impacted Files	Coverage Δ
...rumEventsCollection/resource/resourceCollection.ts	97.22% <80.00%> (-2.78%)	⬇️
...ages/rum-core/src/browser/performanceCollection.ts	14.89% <100.00%> (+1.85%)	⬆️

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