don't build ghost packages

[aaronabramov]

often when we work on a new package and switch between branches, git leaves a lot of empty directories in the directory tree.
Browser build will always crash on empty directories, this condition will make it ignore empty dirs and only build the ones that have package.json

[aaronabramov]

screenshot of the crash

[codecov-io]

Codecov Report

Merging #3934 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3934   +/-   ##
======================================
  Coverage    58.1%   58.1%           
======================================
  Files         196     196           
  Lines        6752    6752           
  Branches        6       6           
======================================
  Hits         3923    3923           
  Misses       2826    2826           
  Partials        3       3

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[github-actions]

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