Added NaN protections

Earlier this week I encountered some rbxm files that appeared to have the values of some NumberSequenceKeypoints as NaN. This would cause the rojo sync to fail as that value was being serialized as nil. Adding a zero here allowed the sync to complete successfully.

rbx_dom_lua/src/EncodedValue.lua

@@ -313,7 +313,7 @@ types = {
 			local keypoints = {}
 
 			for index, keypoint in ipairs(pod.keypoints) do
-				keypoints[index] = NumberSequenceKeypoint.new(keypoint.time, keypoint.value, keypoint.envelope)
+				keypoints[index] = NumberSequenceKeypoint.new(keypoint.time, keypoint.value or 0, keypoint.envelope)
 			end
 
 			return NumberSequence.new(keypoints)