Make vkey witnesses None when empty

[theeldermillenial]

This is a minor fix to vkey witnesses when empty in the transaction builder.

When building a transaction without signing keys, the empty vkey witnesses list gets encoded to an empty value in the witness. This seems to cause an issue for some wallets.

Preferably merged into the chang branch prior to #371

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.25%. Comparing base (043578d) to head (892d610).
Report is 1 commits behind head on chang.

Additional details and impacted files

@@            Coverage Diff             @@
##            chang     #377      +/-   ##
==========================================
+ Coverage   85.22%   85.25%   +0.03%     
==========================================
  Files          32       32              
  Lines        4209     4211       +2     
  Branches     1058     1059       +1     
==========================================
+ Hits         3587     3590       +3     
  Misses        435      435              
+ Partials      187      186       -1     

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