gadget.lua: not crash when Explosion_GetWantedWeaponDef() not defined

same as https://github.com/ZeroK-RTS/Zero-K/commit/da8056b0ed5874dc8778c1d909b74247f8edb3aa commit. 

It will crash gadget.lua when map modder/maker try to use Explosion() event in their gadget.

Example message:  [f=0000560] Error: LuaRules::RunCallIn: error = 2, Explosion, [string "LuaRules/gadgets.lua"]:1768: attempt to call method 'Explosion_GetWantedWeaponDef' (a nil value) 
from ArchShaman.