fix: length of empty byte arrays and dynamic arrays

[tserg]

What I did

Fix #3275.

Also adds DynArray to len() in docs for builtins.

How I did it

Add ~empty case.

How to verify it

See tests

Commit message

fix: length of empty byte arrays and dynamic arrays

Description for the changelog

Fix length of empty byte arrays and dynamic arrays

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[codecov-commenter]

Codecov Report

Merging #3276 (25afda3) into master (d76c6ed) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master    #3276   +/-   ##
=======================================
  Coverage   88.86%   88.86%           
=======================================
  Files          84       84           
  Lines       10599    10599           
  Branches     2214     2214           
=======================================
  Hits         9419     9419           
  Misses        770      770           
  Partials      410      410           

Impacted Files	Coverage Δ
vyper/codegen/core.py	86.74% <100.00%> (+0.04%)	⬆️
vyper/builtins/_utils.py	100.00% <0.00%> (ø)
vyper/builtins/functions.py	90.43% <0.00%> (ø)

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