Hi jeladi
How to solve this problem? I think there are two ways.
1.Set the first_lod of the terrain to avoid loading no data levels. first need to add a debug layer, and get the wms layer min level.
SXEarth is a osgEarth GUI editor, demonstrate the figure.

2.Add a new image layer and configure max level as the wms layer max no data level.
Demo figure
To achieve smooth transition of the two layers, you can use the min_range property of world.tif layer, terrain's attenuation_distance property is also effect. You can try it.
SXEarth,a free osgEarth GUI Editor and Flight Simulation.
Email: seegogo@126.com
SXEarth download:
http://www.sxsim.com/en/