I worried about that with chargers.
My humble take on it is that the quantity of heat....energy in spark x time of occurrence aka power has to exceed the combustion temperature of the grass touching it, be it via a single whack or repetitive whacks in a given amount of time.
One might take combustion temperatures published on wet hay combusting for a temperature. Considering your domestic, home, how water heater (gas at least as I use gas) set to Normal puts out 140F water and you (I) can't leave my hand under the faucet.
That's below combustion temps and I have had the unfortunate experience of sticking my hand in a smoking round bale of SS (that I baled a little too wet and immediately stacked the bales together trapping the heat) and it didn't stay in there but a second.
I can't imagine a charger getting a stalk of JG or such smoking hot. I have been at it 40 years and never heard of anyone starting a fire with a charger, around here or anywhere else, but the theory exists.