feat: send 204 response if null is returned from handler

[danielroe]

#152

Also fixes a bug where a number or boolean value would crash response.

[codecov]

Codecov Report

Merging #154 (5625320) into main (821c286) will increase coverage by 0.00%.
The diff coverage is 40.00%.

❗ Current head 5625320 differs from pull request most recent head 2420229. Consider uploading reports for the commit 2420229 to get more accurate results

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   65.95%   65.95%           
=======================================
  Files          14       14           
  Lines         846      849    +3     
  Branches      175      176    +1     
=======================================
+ Hits          558      560    +2     
  Misses        118      118           
- Partials      170      171    +1     

Impacted Files	Coverage Δ
src/utils/response.ts	58.06% <0.00%> (ø)
src/app.ts	67.81% <50.00%> (-0.03%)	⬇️

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[pi0]

Marking as semver-minor as null usage is rare.