[HDRP][Path Tracing] Quick fix (should have been part of PR 5754 on volumetric scattering)

[eturquin]

One small change got lost somewhere, that should have been part of a recent PR on volumetric scattering (#5754).
It makes sure that the volume coloration, in a similar way to the non-volumetric case, is only applied to non volume-scattering contributions (for which the volume color is set to black, to conserve physicality).

[github-actions]

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[github-actions]

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