sbuffer python enhancements

[guruofquality]

• add nonzero for boolean checking of sbuffer null-ness
• make length zero get call work
• nice exception for calling stuff on a null buffer?

[guruofquality]

https://github.com/guruofquality/gras/commit/901afc2cbf6ac39a186f555789b8c8d8a00d7b07