Change in neo.rawio datatypes

[samuelbray32]

As tracked in this issue, the neo.rawio package has added a buffer_id to multiple of their data types, which cause breaking errors in our code for SpikegadgetsRawIO. Looking at the code, I think we can just add a null value for the buffer stream to resolve

Affected data types:

• _signal_stream_dtype
• _signal_channel_dtype