fix: don't call alloc with a length of 0

[nathanwhit]

Ref denoland/deno#27545.

Fixes a leak in deno.serve with an empty response. We would call to_string_ptr with an empty string and then call alloc with a length of 0 which is UB.

Instead of using alloc directly, which has a large number of invariants to uphold, I've changed to just use Vec APIs. That leaves us with fewer invariants to uphold, handles the tricky cases for us, and we need to make a Vec eventually anyway (since String just wraps a Vec).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 81.89%. Comparing base (0c7f83e) to head (a523dc1).
Report is 216 commits behind head on main.

Files with missing lines	Patch %	Lines
core/runtime/ops.rs	70.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
+ Coverage   81.43%   81.89%   +0.45%     
==========================================
  Files          97      103       +6     
  Lines       23877    27811    +3934     
==========================================
+ Hits        19445    22775    +3330     
- Misses       4432     5036     +604     

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[littledivy]

LGTM