Fix initialization of vector properties when duplicating subgraphs

com.unity.shadergraph/Editor/Data/Graphs/Vector1MaterialSlot.cs

         string[] m_Labels;
 
         public Vector1MaterialSlot()
-        {}
+        {
+            m_Labels = new[] { "X" };
+        }
 
         public Vector1MaterialSlot(
             int slotId,

com.unity.shadergraph/Editor/Data/Graphs/Vector2MaterialSlot.cs

         string[] m_Labels;
 
         public Vector2MaterialSlot()
-        {}
+        {
+            m_Labels = new[] { "X", "Y" };
+        }
 
         public Vector2MaterialSlot(
             int slotId,

com.unity.shadergraph/Editor/Data/Graphs/Vector3MaterialSlot.cs

 
         public Vector3MaterialSlot()
         {
+            m_Labels = new[] { "X", "Y", "Z" };
         }
 
         public Vector3MaterialSlot(