filebeat/input/udp: fix inverted nil check logic

[efd6]

What does this PR do?

This fixes an incorrect nil check that resulted from an incomplete test inversion during development.

Why is it important?

The current code causes a panic in the case that a metric is closed and reused.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @efd6? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[elasticmachine]

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• Start Time: 2022-12-01T23:37:06.657+0000

• Duration: 84 min 41 sec

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Test	Results
Failed	0
Passed	7133
Skipped	738
Total	7871

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