[Sonicwall_firewall] Fix handling of NAT fields

[adriansr]

What does this PR do?

Fixes the logic for handling the different formats for NAT fields in the recently added sonicwall_firewall package.

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.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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• Start Time: 2022-05-25T18:35:44.530+0000

• Duration: 15 min 13 sec

Test stats 🧪

Test	Results
Failed	0
Passed	11
Skipped	0
Total	11

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚 3.538
Classes	100.0% (1/1)	💚 3.538
Methods	100.0% (16/16)	💚 11.542
Lines	98.211% (1098/1118)	👍 8.895
Conditionals	100.0% (0/0)	💚

[adriansr]

/test

[adriansr]

Waiting for #3426 to be merged so that the use of a second IPv6 address is allowed by elastic-package.

[adriansr]

/test