path: refactor more path code for simplicity

1) Consolidate format to a single function.
2) Move some code that can only be reached in some code branches
   that was formerly executed in all cases.
3) Explicitly check for the string length of zero instead of
   converting the string to a boolean.
4) Consolidate nested if statements if possible e.g.,
     if (foo) { if (bar) { /* do stuff */ } }
   to reduce indentation depth.
5) Simplify checks by removing extra length checks when comparing
   two strings.
6) Use object shorthand notation where possible.

PR-URL: https://github.com/nodejs/node/pull/25278


Reviewed-By: Michaël Zasso <targos@protonmail.com>