private T applyEmptyCtorOptimization() {
        return Objects.requireNonNull(emptyCtorOptimization.get(),
                "BuilderRef 'emptyCtorOptimization' must not return null");
    }
Default implementation of {@link BuilderRef}.

<p>This class is not threadsafe; it should only be used to build up
other objects using {@link #copy()}.

@param <T> The type of value being built, borrowed, and copied.
smithy-utils/src/main/java/software/amazon/smithy/utils/DefaultBuilderRef.java