[now-routing-utils] Fix validation for routes/cleanUrls

[styfle]

This fixes an issue when routes is normalized to null which should be treated the same as undefined.

[codecov-io]

Codecov Report

Merging #3211 into canary will increase coverage by 2.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3211      +/-   ##
==========================================
+ Coverage   11.47%   13.55%   +2.08%     
==========================================
  Files         268      268              
  Lines       10096    10354     +258     
  Branches     1313     1217      -96     
==========================================
+ Hits         1159     1404     +245     
- Misses       8847     8888      +41     
+ Partials       90       62      -28

Impacted Files	Coverage Δ
src/util/metrics.ts	68.18% <0%> (-2.41%)	⬇️
src/util/dev/router.ts	82.35% <0%> (+3.28%)	⬆️
src/util/dev/builder-cache.ts	80.74% <0%> (+3.47%)	⬆️
src/util/projects/get-project-by-id-or-name.ts	28.57% <0%> (+3.57%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/dev/server.ts	64.8% <0%> (+5.56%)	⬆️
src/util/dev/builder.ts	78.32% <0%> (+5.59%)	⬆️
src/util/prefer-v2-deployment.ts	61.9% <0%> (+5.65%)	⬆️
src/util/output/create-output.ts	66.66% <0%> (+7.29%)	⬆️

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