`None` input to a `String` field raise an `TypeError`

[samuelcolvin]

Error is:

   await self.write_items(prepared)
  File "/usr/local/lib/python3.11/site-packages/asynch/proto/columns/stringcolumn.py", line 13, in write_items
    await self.writer.write_strings(items)
  File "/usr/local/lib/python3.11/site-packages/asynch/proto/streams/buffered.py", line 49, in write_strings
    await self.write_varint(len(packet))
                            ^^^^^^^^^^^
TypeError: object of type 'NoneType' has no len()

[Wally110]

I encountered the same issue.

[long2ice]

Use Nullable(String) instead of String