HDRenderPipeline: Fix issue with Velocity buffer being null when motion vector are disabled

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

                     cmd.GetTemporaryRT(HDShaderIDs._CameraDepthTexture, hdcamera.actualWidth, hdcamera.actualHeight, m_CameraDepthStencilBuffer.rt.depth, FilterMode.Point, m_CameraDepthStencilBuffer.rt.format);
                     m_CopyDepth.SetTexture(HDShaderIDs._InputDepth, m_CameraDepthStencilBuffer);
                     cmd.Blit(null, HDShaderIDs._CameraDepthTexture, m_CopyDepth);
-                    cmd.GetTemporaryRT(HDShaderIDs._CameraMotionVectorsTexture, hdcamera.actualWidth, hdcamera.actualHeight, 0, FilterMode.Point, m_VelocityBuffer.rt.format);
-                    HDUtils.BlitCameraTexture(cmd, hdcamera, m_VelocityBuffer, HDShaderIDs._CameraMotionVectorsTexture);
+                    if (m_VelocityBuffer != null)
+                    {
+                        cmd.GetTemporaryRT(HDShaderIDs._CameraMotionVectorsTexture, hdcamera.actualWidth, hdcamera.actualHeight, 0, FilterMode.Point, m_VelocityBuffer.rt.format);
+                        HDUtils.BlitCameraTexture(cmd, hdcamera, m_VelocityBuffer, HDShaderIDs._CameraMotionVectorsTexture);
+                    }
                     cmd.GetTemporaryRT(HDShaderIDs._CameraColorTexture, hdcamera.actualWidth, hdcamera.actualHeight, 0, FilterMode.Point, m_CameraColorBuffer.rt.format);
                     HDUtils.BlitCameraTexture(cmd, hdcamera, m_CameraColorBuffer, HDShaderIDs._CameraColorTexture);
                     source = HDShaderIDs._CameraColorTexture;