In considering the National Security of the United States, should NASA be concerned about what we may encounter within interstellar space?

You beat me to my point. We have exactly one spacecraft which left from this planet and reached interstellar space, it it took almost 40 years to get that far. Anyone who shows up here, either exceeded the speed of light, invented a functional warp drive, or came in a generation ship or cold sleep. In short, either they are castaways with no way to return, or are vastly ahead of us in terms of technology.

If the aliens from Independence Day do show up to make us into their food source, we;ll just have to muddle through as best we can. No point in designating a few billion dollars to preparing for a scenario that's unlikely in the extreme, but I have my ammo cabinet stocked ... just in case.