public static <T> ToDoubleNullable<T> intToDoubleNullable(ToIntNullable<T> expression) {
        return expression.mapToDoubleIfPresent(b -> b);
    }
Creates and returns an expression that converts the result of the
specified expression into a {@code double} by casting. If the result of
the input is {@code null}, then the new expression will also return
{@code null}.

@param expression  the initial expression
@param <T>         the input type
@return            the new expression
runtime-parent/runtime-compute/src/main/java/com/speedment/runtime/compute/expression/Expressions.java