[GCP] Update null checks and if checks in the rename

[ishleenk17]

• Bug

What does this PR do?

This PR adds the if check for event.original and the ignore_missing check for the renaming of message to event.original
The packages where the changes are made are as below:

• loadbalancing_logs
• vpcflow
• firewall
• audit
• dns

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.

Related issues

• Relates Add checks to rename processor for the message field for all Observability Integrations #7822

[elasticmachine]

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• Start Time: 2023-09-22T06:36:04.635+0000

• Duration: 19 min 39 sec

Test stats 🧪

Test	Results
Failed	0
Passed	68
Skipped	0
Total	68

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (6/6)	💚
Files	100.0% (6/6)	💚 1.471
Classes	100.0% (6/6)	💚 1.471
Methods	87.826% (101/115)	👎 -9.858
Lines	96.013% (1469/1530)	👍 2.384
Conditionals	100.0% (0/0)	💚

[zmoog]

LGTM.

[kcreddy]

LGTM 👍🏼

[elasticmachine]

Package gcp - 2.29.1 containing this change is available at https://epr.elastic.co/search?package=gcp