fix[codegen]: fix bytes copying routines for 0-length case

[charles-cooper]

What I did

per title. per #4645 and #4644 (and their associated security advisories) it is difficult, but not impossible, to construct an empty bytestring which has side effects. for hygiene, don't fastpath out the side effects for zero-length bytestrings

How I did it

How to verify it

Commit message

per title. as in the referenced pull requests and security advisories,
it is difficult, but not impossible, to construct an empty bytestring
which has side effects. in this commit, don't fastpath out the side
effects for zero-length bytestrings

references:
- https://github.com/vyperlang/vyper/pull/4644
- https://github.com/vyperlang/vyper/security/advisories/GHSA-qhr6-mgqr-mchm
- https://github.com/vyperlang/vyper/pull/4645
- https://github.com/vyperlang/vyper/security/advisories/GHSA-3vcg-j39x-cwfm

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.53%. Comparing base (1fdfd69) to head (f7940f0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/codegen/core.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4649      +/-   ##
==========================================
- Coverage   92.53%   92.53%   -0.01%     
==========================================
  Files         129      129              
  Lines       18593    18604      +11     
  Branches     3225     3228       +3     
==========================================
+ Hits        17205    17215      +10     
- Misses        942      943       +1     
  Partials      446      446              

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[cyberthirst]

i will just add context for future me.. we added this if statement to make some fast-path-out optimizations easier to detect. e.g. in make_bytearray_coppier we now directly store to destination if we detect empty without having to load anything

[cyberthirst]

let's just merge charles-cooper#86 and we should be good to go