Fixed nullref when Destroy gets called repeatedly

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/RenderPass/Upscalers/FSR3/Fsr3UpscalerContext.cs

@@ -125,8 +125,11 @@ namespace FidelityFX
             DestroyConstantBuffer(ref _spdConstantsBuffer);
             DestroyConstantBuffer(ref _upscalerConstantsBuffer);
 
-            _commandBuffer.Dispose();
-            _commandBuffer = null;
+            if (_commandBuffer != null)
+            {
+                _commandBuffer.Dispose();
+                _commandBuffer = null;
+            }
         }
 
         public void Dispatch(Fsr3Upscaler.DispatchDescription dispatchParams)