Which issue, if any, is this issue related to?

Closes #3401

Is there anything in the PR that needs further explanation?

This commit changes the following:

1. To avoid waiting for stdin without any input. For example, just running the stylelint command
   immediately exits, instead of waiting. I believe this behavior is more user-friendly.
2. To avoid different outputs on empty files and empty stdin. This is achieved by allowing an empty
   string input ('') from stdin. See the example below:

   # code is undefined
   bin/stylelint.mjs
   
   # code is ''
   echo -n '' | bin/stylelint.mjs

See also process.stdin.isTTY.