Try to fix compiler crash when running JSONTest

[sebsto]

Back to the original form #expect(json["non_existent"] == nil

It doesn't crash on

swift --version                                            
swift-driver version: 1.127.14.1 Apple Swift version 6.2 (swiftlang-6.2.0.19.9 clang-1700.3.19.1)
Target: arm64-apple-macosx26.0

It crashes on

swift --version 
Swift version 6.2 (swift-6.2-RELEASE)
Target: aarch64-unknown-linux-gnu