atlassian_{bitbucket,confluence,jira}: fix handling of zero-length event lists

[efd6]

What does this PR do?

This drops messages that have a zero-length event list for the atlassian packages.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes atlassian_*: packages do not gracefully handle messages with no event #4712

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• Start Time: 2022-11-24T00:57:24.285+0000

• Duration: 17 min 52 sec

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Test	Results
Failed	0
Passed	29
Skipped	0
Total	29

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (3/3)	💚
Files	100.0% (7/7)	💚
Classes	100.0% (7/7)	💚
Methods	100.0% (62/62)	💚 75.0
Lines	98.778% (1212/1227)	👎 -1.222
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Package atlassian_bitbucket - 1.5.1 containing this change is available at https://epr.elastic.co/search?package=atlassian_bitbucket

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Package atlassian_confluence - 1.6.1 containing this change is available at https://epr.elastic.co/search?package=atlassian_confluence

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Package atlassian_jira - 1.6.1 containing this change is available at https://epr.elastic.co/search?package=atlassian_jira