fix(lambda-at-edge): serve HTML pages with no props (i.e static pages) properly on preview mode enabled

[dphang]

• When preview mode was enabled these HTML pages with no props were trying to be rendered via SSR but they don't exist in ssr.nonDynamic object in the build manifest. Hence the router was returning pages/404.html and then the handler was trying to require this, leading to a 503.

I don't have e2e tests for preview mode but we can add it in the near future.

[codecov]

Codecov Report

Merging #701 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #701   +/-   ##
=======================================
  Coverage   80.18%   80.18%           
=======================================
  Files          54       54           
  Lines        1731     1731           
  Branches      366      366           
=======================================
  Hits         1388     1388           
  Misses        285      285           
  Partials       58       58           

Impacted Files	Coverage Δ
...ackages/libs/lambda-at-edge/src/default-handler.ts	95.31% <100.00%> (ø)

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