Add config check to replace processor

[fearful-symmetry]

Proposed commit message

fixes #38747

This adds a validation method to the config for the replace processor, so we don't get a null pointer deref and explosion in cases where a user hasn't configured a regex pattern.

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.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

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

• The upcoming major version of the Elastic Stack
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To fixup this pull request, you need to add the backport labels for the needed
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• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[efd6]

I think this may be overly strict. The implementation here prevents the use of the replace processor for removing a pattern; it is not possible to express s/<pattern>// while it was previously. The conventional approach of putting the value on a pointer to allow differentiation between an absence and a zero does not work with ucfg, it seems, so the validation would have to be done in code rather than in the struct tag.