[8.0](backport #30057) libbeat/reader: Fix messge conversion to beat.Event

[mergify]

This is an automatic backport of pull request #30057 done by Mergify.

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[elasticmachine]

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• Reason: null

• Start Time: 2022-01-28T09:52:29.825+0000

• Duration: 147 min 30 sec

• Commit: 587bc85

Test stats 🧪

Test	Results
Failed	0
Passed	47580
Skipped	4285
Total	51865

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