One might argue encountering something like this would be more akin to a natural catastrophe (like a killer meteor) rather than an alien invasion.
Stephen Baxter wrote a novel in which galaxies would be regularly wiped out by a natural phenomena, and that explains why there are no aliens in his universe. It's "Manifold: Space" if you're interested.
Life as we know it is just chemical reactions occuring within some membrane or walled environment. I don’t see why that definition couldn’t be extended to self sustaining chemical reactions that aren’t bound by a membrane or wall structure. The purpose of that membrane or wall is to just establish favorable conditions for the chemistry within the cell. If conditions could be established without such a membrane there wouldn’t be a reason for it. If reactions were energetically favorable across a wide spectrum of conditions there wouldn’t be a need for it either.
There are some that consider fire to be a form of life. I don’t think that is such a wrong idea if we are to consider how wide the scope of possibilities are in the universe.
Stephen Baxter wrote a novel in which galaxies would be regularly wiped out by a natural phenomena, and that explains why there are no aliens in his universe. It's "Manifold: Space" if you're interested.