[unity] Don't clear empty prefab MeshFilters.

spine-unity/Assets/spine-unity/Editor/SkeletonRendererInspector.cs

@@ -170,7 +170,7 @@ namespace Spine.Unity.Editor {
 					#if NO_PREFAB_MESH
 					if (isInspectingPrefab) {
 						MeshFilter meshFilter = component.GetComponent<MeshFilter>();
-						if (meshFilter != null)
+						if (meshFilter != null && meshFilter.sharedMesh != null)
 							meshFilter.sharedMesh = null;
 					}
 					#endif