Fix crash whem matching near spawnpoint with Zuma generation

One of the checks would fetch the previous group's last sphere, but that group could have no spheres due to it being a generator group. This would cause a crash.

src/SphereGroup.lua

@@ -556,7 +556,8 @@ function SphereGroup:shouldMatch(position)
 	end
 	if self.config.permitLongMatches then
 		-- if is magnetizing with previous group and we want to maximize the count of spheres
-		if self.prevGroup and not self.prevGroup.delQueue and _Game.session:colorsMatch(self.prevGroup:getLastSphere().color, self.spheres[1].color) and position1 == 1 then
+		-- We are doing a check whether that previous group is empty. It CAN be empty, if the Zuma sphere generation is enabled.
+		if self.prevGroup and not #self.prevGroup.spheres == 0 and not self.prevGroup.delQueue and _Game.session:colorsMatch(self.prevGroup:getLastSphere().color, self.spheres[1].color) and position1 == 1 then
 			return false
 		end
 		-- same check with the next group