Fix #1970: stl with empty solid

Log warning instead of stopping whole import.

code/STLLoader.cpp

@@ -352,7 +352,7 @@ void STLImporter::LoadASCIIFile( aiNode *root ) {
 
         if (positionBuffer.empty())    {
             pMesh->mNumFaces = 0;
-            throw DeadlyImportError("STL: ASCII file is empty or invalid; no data loaded");
+            ASSIMP_LOG_WARN("STL: mesh is empty or invalid; no data loaded");
         }
         if (positionBuffer.size() % 3 != 0)    {
             pMesh->mNumFaces = 0;