[now-build-utils] Allow a null fallback in Prerender

[Timer]

This allows a null fallback to be provided to a Prerender. The use case is a lazily prerendered route (often meaning dynamically rendered).

[codecov-io]

Codecov Report

Merging #3144 into canary will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           canary   #3144      +/-   ##
=========================================
+ Coverage   13.29%   13.3%   +<.01%     
=========================================
  Files         268     268              
  Lines       10332   10333       +1     
  Branches     1218    1217       -1     
=========================================
+ Hits         1374    1375       +1     
  Misses       8895    8895              
  Partials       63      63

Impacted Files	Coverage Δ
src/util/dev/builder.ts	76.1% <0%> (-0.83%)	⬇️
src/commands/dev/index.ts	0% <0%> (ø)	⬆️

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