Fix 2D TF with MultiVolume

Fix for #18527 (closed)

Providing (only) 2D TFs to a MultiVolume provokes a crash in vtkOpenGLGPUVolumeRayCastMapper, the shader doesn't compile. The reason for this is that in computeLighting, the shader composer only looks at the volume property of the first volume, so when only 2D TFs are given, the gradient is used but not defined. This fix ensures that it is always defined when needed with MultiVolume.