Don't ever do that. With the plug out, the 'stuff' that comes out of the plug hole is essentially a fuel-air explosive (FAE), just looking for an ignition source.
I've been doing this for decades and never had one catch on fire. Not even a flame thrower.
If you ever have a plug blow out THEN you should remove the coil or at least unplug the connector to it or much better yet, unplug the fuel injector for that cylinder so you're not washing away the oil and flooding out the cylinder and creating a possibility for a fire. But starting an engine with a spark plug out there so you can sit there and let it run all day and with no ignition Source there it's not going to catch on fire. Not even a vapor Mist really comes out. I only do it for 3 or 4 seconds to blow everything or anything out of the cylinder.