Distortion::RefreshCompositor check nonzero params

[scpeters]

There is a small regression from the fix in #3060 when a camera has a <distortion/> block containing k and p parameters with all 0 values like the following:

            <distortion>
              <k1>0</k1>
              <k2>0</k2>
              <k3>0</k3>
              <p1>0</p1>
              <p2>0</p2>
              <center>0.5 0.5</center>
            </distortion>

In this case, the Distortion::SetCamera method will be called because the camera contains a <distortion/> element, but SetCamera will return early without creating a compositor instance. For consistency, add the same check to the RefreshCompositor method, otherwise a compositor will be added without any of the configuration done in SetCamera.