vtkGLTFExporter: fixed writing of bin files

When FileName does not contain a path, e.g. FileName = 'my_file.gltf'

IO/Export/vtkGLTFExporter.cxx

@@ -142,7 +142,10 @@ void WriteBufferAndView(vtkDataArray *inda, const char *fileName, bool inlineDat
 
     std::string fullPath =
       vtksys::SystemTools::GetFilenamePath(fileName);
-    fullPath += "/";
+    if (fullPath.size() > 0)
+    {
+      fullPath += "/";
+    }
     fullPath += result;
 
     // now write the data