feat(wkt): NullValue behaves like enums

[coryan]

In some contexts it is useful if NullValue behaves more like an enum.
Protobuf defines it as an enum, and the generator produces code assuming
it has the interface defined for enums.

Part of the work for #1384

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.95%. Comparing base (4040e0b) to head (259860d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
+ Coverage   95.87%   95.95%   +0.07%     
==========================================
  Files          38       38              
  Lines        1526     1531       +5     
==========================================
+ Hits         1463     1469       +6     
+ Misses         63       62       -1     

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