🐛 Fix empty path deletion on iOS

[vinceglb]

Summary

• Treat an empty NSURL as an empty Kotlin path on Apple platforms.
• Preserve FileKitNSURLNullPathException for other pathless URLs.
• Add regression coverage for PlatformFile("").delete(mustExist = false).

This makes empty-path deletion consistent with Android when mustExist is false.

Fixes #592

Verification

• Targeted iOS simulator regression test
• ./gradlew :filekit-core:check
• Kotlin lint
• git diff --check

[vinceglb]

@codex review this

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid running this iOS-only regression on JVM

This test lives in the shared nonWebTest source set, which is wired into jvmTest as well as native tests. On JVM, PlatformFile("") wraps java.io.File(""), i.e. the current working directory; because it exists, delete(mustExist = false) still attempts File.delete() and a non-empty checkout returns false, causing IOException("Deletion failed") and breaking :filekit-core:jvmTest/check. Please scope this regression to Apple/native or make JVM handle empty paths consistently before sharing the test.

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