Fix SDO writes of empty strings

[willson556]

Previously, when attempting a write of "" to a VISIBLE_STRING, the expedited SDO request would not be sent and the transaction would timeout.

I traced this down to the BufferedWriter not issuing a write to the underlying stream when it has only been called with b"". Additionally, it turns out that it's not possible to correctly send an expedited SDO request with size == 0.

To resolve this, if the transaction size is known to be 0, a segmented transfer is used.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.36%. Comparing base (ffbd10f) to head (79790d9).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #551   +/-   ##
=======================================
  Coverage   71.36%   71.36%           
=======================================
  Files          26       26           
  Lines        3129     3129           
  Branches      480      480           
=======================================
  Hits         2233     2233           
  Misses        765      765           
  Partials      131      131           

Files with missing lines	Coverage Δ
canopen/sdo/client.py	72.01% <100.00%> (ø)

[acolomb]

Nice find and good job in fixing it. Some nit-picks, but nothing serious that would keep this from being merged.

[willson556]

Nice find and good job in fixing it. Some nit-picks, but nothing serious that would keep this from being merged.

Thanks for the quick review! Got back around to this today and realized you were right about the command byte issue. I updated the code to force a segmented transfer instead since the 2-bit size field overflows when attempting to use a transfer size of 0.