When I was at JPL for the Mars Polar Lander landing I raised the same question with several people including Laurie Leshin and I was told by all that is wasn't a problem because the products of decomposition are not the sort of things to worry about; nitrogen, hydrogen and ammonia.

I thought, and still think, ammonia might even go liquid or icy when it gets chilly, but I haven't looked into it in detail as regards temps and pressures. Of course I wondered whether hot ammonia might have some chemical effect, but I suppose that in such a low pressure environment (for us on Earth it's almost 99% vacuum) the blast effect would be minimal because the gas would disperse as soon as it leaves the nozzle.

Marcus