Adds ifFilled utility method to Strings

[idlira]

Additional Notes

• This PR fixes or works on following ticket(s): OX-12357

Checklist

• Code change has been tested and works locally
• Code was formatted via IntelliJ and follows SonarLint & best practices
• Patch Tasks: Is local execution of Patch Tasks necessary? If so, please also mark the PR with the tag.

[Copilot]

Pull request overview

Adds a small utility to Strings to execute a callback only when an input value is considered “filled” (non-null and non-empty via toString()), with accompanying unit tests to validate the new behavior.

Changes:

• Added Strings.ifFilled(@Nullable Object, Consumer<String>) helper.
• Added Kotlin tests covering ifFilled behavior for filled/empty/null values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/sirius/kernel/commons/Strings.java	Introduces the ifFilled utility method using isFilled to guard callback execution.
src/test/kotlin/sirius/kernel/commons/StringsTest.kt	Adds unit tests for the new Strings.ifFilled method across several input types.

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