Add a minimum value when calculing the height blend factors to avoid a division by zero when all layers are set to zero in the layer mask map.

com.unity.render-pipelines.high-definition/HDRP/Material/LayeredLit/LayeredLitData.hlsl

 
     // Normalize
     maxHeight = GetMaxHeight(maskedHeights);
-    maskedHeights = maskedHeights / maxHeight.xxxx;
+    maskedHeights = maskedHeights / max(maxHeight.xxxx, 1e-6);
 
     return maskedHeights.yzwx;
 }