When soldering service stops or caps next to drywall, before I fire up the torch, I take one of those plastic oil bottles with an extendable spout that I have previously removed the oil and cleaned well. Then I put clean water in the bottle.
There is usually enough space around the pipe coming out of the wall for the small extender tube to fit into. I squirt in a small amount of water — not enough to create mold, but just enough to wet the inside of the drywall to keep it safe during soldering.