Fix ReadOnlyArrayMap.containsValue to handle null

[NithinU2802]

Previously, containsValue() assumed stored values were non-null when comparing values, which could result in incorrect behavior when the map contained a null value. The comparison has been updated to be null-safe while preserving the existing behavior for non-null values.

I'm happy to make any changes if needed.

Closes #8651

[linux-foundation-easycla]

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[jack-berg]

Looks like a formatting failure. Please run ./gradlew spotlessApply and commit / push.

https://github.com/open-telemetry/opentelemetry-java/blob/main/docs/knowledge/build.md