fix: Avoid allocation in the stringifyValue

[alexandear]

The PR refactors implementation of the stringifyValue function.

w.Write([]byte("<nil>")) may allocate, better is to use w.WriteString("<nil>")

[gmlewis]

Thank you, @alexandear !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.97%. Comparing base (2b8c7fa) to head (47271cf).
Report is 123 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3292      +/-   ##
==========================================
- Coverage   97.72%   92.97%   -4.75%     
==========================================
  Files         153      171      +18     
  Lines       13390    11715    -1675     
==========================================
- Hits        13085    10892    -2193     
- Misses        215      729     +514     
- Partials       90       94       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[air-hand]

LGTM

[gmlewis]

Thank you, @air-hand !
Merging.