[Filebeat] decode_cef - allow MACs without separators

[andrewkroh]

What does this PR do?

Accept MAC addresses that do not contain separators (i.e. 000D60AF1B61).

Fixes #27050

Why is it important?

Some log sources do not comply with the CEF spec and send valid MACs but without separators. The processor should identify these and accept them.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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.

Related issues

• Closes Decode_CEF preprocessor fails when MAC address is without separators #27050

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-07-28T15:55:00.262+0000

• Duration: 100 min 7 sec

• Commit: adb8779

Test stats 🧪

Test	Results
Failed	0
Passed	7881
Skipped	1201
Total	9082

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	7881
Skipped	1201
Total	9082

[LaZyDK]

LGTM