Avoid trying to adjust dropped events on test results

[jsoriano]

If pipelines drop events during pipeline tests, they are set to nil, don't try to process them in any way to avoid failures.

elastic-package test pipeline reports this error:

ERROR: verifying test result failed: comparing test results failed: can't adjust test results: can't unmarshal event: unexpected end of JSON input 

Needed for elastic/integrations#1145

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #397 updated

• Start Time: 2021-06-30T07:50:14.717+0000

• Duration: 21 min 45 sec

• Commit: cd7e387

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Test	Results
Failed	0
Passed	315
Skipped	2
Total	317

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[mtojek]

This is an unfortunate use case where we need the "nil" value. Thank you for the fix.

[jsoriano]

This is an unfortunate use case where we need the "nil" value. Thank you for the fix.

Well, this allows to validate that certain events are dropped, but it is true that something more explicit would be better.