Fixes #683 Allow empty PublicKey

[sappenin]

• Allow an empty PublicKey to be built using an empty builder
• Add unit tests to verify functionality.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.53%. Comparing base (4c1b8e8) to head (6b4d588).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #684   +/-   ##
=========================================
  Coverage     92.53%   92.53%           
- Complexity     2103     2104    +1     
=========================================
  Files           432      432           
  Lines          5735     5737    +2     
  Branches        510      510           
=========================================
+ Hits           5307     5309    +2     
  Misses          280      280           
  Partials        148      148           

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[sappenin]

After considering this further, we should remove the empty builder so that developers don't accidentally omit an actual value. Instead, we should update EMPTY_PUBLIC_KEY to be this:

PublicKey EMPTY_PUBLIC_KEY = PublicKey.builder().value(UnsignedByteArray.empty()).build();

That way, if someone wants to overtly use an empty public key, it will be obvious they are do as much.