null
fun <T> nullToFailure( value: T?, key: Metadata.Key<T>, optional: Boolean): Validation<T>
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For a non-existing attribute, a Scala Expression evaluates to a Failure, which may then be ignored in com.github.phisgr.gatling.grpc.HeaderPair.mutateMetadata.
Translating to the Java API, a Failure corresponds to throwing an exception. But for a simple null value we can skip the throw and catch.