Fix perf regression caused by change 2248004

- Basically when usdImaging is getting the material path to track by calling
  UsdShadeMaterialBindingAPI:ComputeBoundMaterial, 2248004 was always
  using the winning binding to get the target path, irrespective of whether the
  bound material exists or not. But we can do better and simply return the path
  of the bound material if it exists, instead of always getting the winning
  relationship targets (which is useful when the bound target doesn't exist
  on the loaded stage).

(Internal change: 2248431)

pxr/usdImaging/usdImaging/resolvedAttributeCache.h

@@ -704,8 +704,12 @@ struct UsdImaging_MaterialStrategy {
     // inherited path to bound target
     // depending on the load state, override, etc bound target path might not be
     // queried as a UsdShadeMaterial on the stage.
-    typedef SdfPath value_type;         // inherited path to bound target
-    typedef UsdRelationship query_type; // Hold the winning relationship binding
+
+    // inherited path to bound target
+    typedef SdfPath value_type;         
+    // Hold the computed path of the bound material or target path of the
+    // winning material binding relationship
+    typedef SdfPath query_type; 
 
     using ImplData = UsdImaging_MaterialBindingImplData;
 
@@ -720,12 +724,21 @@ struct UsdImaging_MaterialStrategy {
         ImplData *implData) 
     {
         UsdRelationship bindingRel;
-        UsdShadeMaterialBindingAPI(prim).ComputeBoundMaterial(
-            &implData->GetBindingsCache(), 
-            &implData->GetCollectionQueryCache(),
-            implData->GetMaterialPurpose(),
-            &bindingRel);
-        return bindingRel;
+        UsdShadeMaterial materialPrim = 
+            UsdShadeMaterialBindingAPI(prim).ComputeBoundMaterial(
+                &implData->GetBindingsCache(), 
+                &implData->GetCollectionQueryCache(),
+                implData->GetMaterialPurpose(),
+                &bindingRel);
+
+        if (materialPrim) {
+            return materialPrim.GetPath();
+        }
+
+        const SdfPath targetPath =
+            UsdShadeMaterialBindingAPI::GetResolvedTargetPathFromBindingRel(
+                    bindingRel);
+        return targetPath;
     }
 
     static 
@@ -736,19 +749,15 @@ struct UsdImaging_MaterialStrategy {
     { 
         TF_DEBUG(USDIMAGING_SHADERS).Msg("Looking for \"preview\" material "
                 "binding for %s\n", prim.GetPath().GetText());
-        if (*query) {
-            const SdfPath targetPath = 
-                UsdShadeMaterialBindingAPI::GetResolvedTargetPathFromBindingRel(
-                    *query);
-            if (!targetPath.IsEmpty()) {
-                return targetPath;
-            }
-        }
+
         // query already contains the resolved material binding for the prim. 
         // Hence, we don't need to inherit the binding from the parent here. 
         // Futhermore, it may be wrong to inherit the binding from the parent,
         // because in the new scheme, a child of a bound prim can be unbound.
-        return value_type();
+        //
+        // Note that query could be an empty SdfPath, which is the default
+        // value.
+        return *query;
     }
 
     static