`cudf::size_of` throws when the column type is STRING

[kevkrist]

cuCascade/src/data/gpu_data_representation.cpp

Line 38 in b657211

total_size += static_cast<std::size_t>(col.size()) * cudf::size_of(col.type());

I know that this is a placeholder implementation, but when the column time is STRING, cudf::size_of() throws an exception, invoking get_size_in_bytes() breaks in that case.