fix: empty builtin in kwargs and constants

[tserg]

What I did

Fix #3004. Also fixes empty for constant.

How I did it

Add _kwargable attribute to Empty builtin.

How to verify it

See tests.

Commit message

fix: allow empty builtin in kwargs and constants

Description for the changelog

Allow empty builtin in kwargs and constants.

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[codecov-commenter]

Codecov Report

Merging #3008 (3e33543) into master (f31f0ec) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master    #3008   +/-   ##
=======================================
  Coverage   88.29%   88.30%           
=======================================
  Files          97       97           
  Lines       10919    10926    +7     
  Branches     2583     2585    +2     
=======================================
+ Hits         9641     9648    +7     
  Misses        830      830           
  Partials      448      448           

Impacted Files	Coverage Δ
vyper/semantics/types/utils.py	90.22% <83.33%> (+0.46%)	⬆️
vyper/builtin_functions/functions.py	90.41% <100.00%> (+<0.01%)	⬆️

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[charles-cooper]

slept on it and i think _kwargable is a fine name for this (as in the terminology in vyper/semantics/types/utils.py, kwargable implies constant)

[charles-cooper]

confirmed privately this PR is ready to come out of draft