COMP: Fix return error in MeshIO::write

System: gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

error: converting to ‘bool’ from ‘std::nullptr_t’ requires
direct-initialization

Base/Geometry/Reader/imstkMeshIO.cpp

@@ -134,7 +134,7 @@ MeshIO::write(const std::shared_ptr<imstk::Mesh> imstkMesh, const std::string& f
     }
 
     LOG(WARNING) << "MeshIO::write error: file type not supported";
-    return nullptr;
+    return false;
 }
 
 } // imstk