feat: add reportIfNil to report unexpected nil values

[Muhammed9991]

Add reportIfNil: Report unexpected nil values as issues

This PR introduces a new utility, reportIfNil, to the swift-issue-reporting library. It evaluates an optional value and reports an issue if the value is nil, using the existing IssueReporter infrastructure.

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Motivation

While withErrorReporting handles thrown errors, there is currently no equivalent for reporting unexpected nil values — a common source of bugs in Swift.

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Tests Included

Tests are split into focused cases for better clarity and maintainability:

• ✅ Reports issue for nil value with default message
• ✅ Reports issue for nil value with custom message
• ✅ Does not report issue when value is non-nil

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Related

Complements existing withErrorReporting functionality by offering a similar utility for handling optional values that shouldn't be nil.