fix(Runtime): Address edge case where GetNumberOfTransformsRetargeted is called too early

Summary:
If `GetNumberOfTransformsRetargeted` is called too early, then `TargetSkeletonData` might be `null` or `TargetSkeletonData.BodyToBoneData` might be `null`.

An example edge case would be:
1. Application starts up while scripts are initializing.
2. A permissions check appears. The application pauses and `AnimationRigSetup` calls `DisableRigAndUpdateState` via `OnApplicationFocus` when `hasFocus` is false.
3. `DisableRigAndUpdateState` calls `_retargetingLayer.GetNumberOfTransformsRetargeted()`, and `_retargetingLayer` metadata might not be ready at that point.

Reviewed By: andkim-meta

Differential Revision: D46335611

fbshipit-source-id: 6f60a3bb9134a227cddcfe99730abedf9aa3b307

Author: sohailshafiiWk

Runtime/Scripts/AnimationRigging/RetargetingLayer.cs

@@ -175,6 +175,11 @@ public bool EnableTrackingByProxy
         public int GetNumberOfTransformsRetargeted()
         {
             int numTransforms = 0;
+            // return default case if this is called before initialization.
+            if (TargetSkeletonData == null || TargetSkeletonData.BodyToBoneData == null)
+            {
+                return numTransforms;
+            }
             foreach (var boneData in TargetSkeletonData.BodyToBoneData.Values)
             {
                 if (boneData.CorrectionQuaternion != null)