Backport #868 to 7.4 (custom pivot geter/setter) (#869)

This PR backports some changes to rbx_dom_lua to fix serving model
pivots

plugin/rbx_dom_lua/EncodedValue.lua

@@ -493,9 +493,32 @@ types = {
 	},
 }
 
+types.OptionalCFrame = {
+	fromPod = function(pod)
+		if pod == nil then
+			return nil
+		else
+			return types.CFrame.fromPod(pod)
+		end
+	end,
+
+	toPod = function(roblox)
+		if roblox == nil then
+			return nil
+		else
+			return types.CFrame.toPod(roblox)
+		end
+	end,
+}
+
 function EncodedValue.decode(encodedValue)
 	local ty, value = next(encodedValue)
 
+	if ty == nil then
+		-- If the encoded pair is empty, assume it is an unoccupied optional value
+		return true, nil
+	end
+
 	local typeImpl = types[ty]
 	if typeImpl == nil then
 		return false, "Couldn't decode value " .. tostring(ty)