public void ifPresent(@Nonnull Consumer<? super T> consumer) {
        if (isPresent()) {
            consumer.accept(this.value);
        }
    }
If a value is present, invoke the specified consumer with the value,
otherwise do nothing.

@param consumer block to be executed if a value is present
@throws NullPointerException if value is present and {@code consumer} is
null
helper/src/main/java/me/lucko/helper/utils/NullableOptional.java