chore(codebase): add nilnil return check

[ivankatliarchuk]

Description

codebase uniformity benefits

1. Reduced Cognitive Load During Code Review: When a codebase adheres to consistent style guidelines, naming conventions, and architectural patterns, reviewers can focus more on the logic and potential issues within the code changes rather than spending time deciphering unfamiliar or inconsistent formatting and structures. This allows for more efficient and insightful reviews, ultimately leading to higher quality code.
2. Enhanced Readability: A uniform codebase is more predictable, making it easier for developers to follow and understand the code, thereby reducing the cognitive load.
3. Easier Onboarding: When a codebase is consistent, new team members can get up to speed more quickly as they don’t have to adjust to different coding styles or paradigms in different parts of the codebase.
4. Efficient Debugging and Maintenance: Uniformity in the codebase makes it easier to spot errors and maintain the code, as we know what to expect and where to look for potential issues.

Added nill-nill check.

• This check reveal few issues
• Removed duplicated code
• Increased coverage for areas where code removed
• clouflfare.go, fqdn.go and istio_* they have nil,nil return, that I'm not sure how to fix yet. Will try to fix in follow-up

Failed run https://github.com/kubernetes-sigs/external-dns/actions/runs/15087988069/job/42412985614?pr=5415

coredns before

coredns with this pr

fixed shadowing

Checklist

• Unit tests updated
• End user documentation updated

[ivankatliarchuk]

/label tide/merge-method-squash

[mloiseleur]

/lgtm
/approve

[k8s-ci-robot]

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This pull-request has been approved by: mloiseleur

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