path: refactor for less indentation

This moves the `if (len === 1)` case to the top of the function.
That way it is possible to reduce the indentation level due to
returning early in that case.

On top of that the following was done:

1) For clarity refactored for loops which were meant to count up a
   variable into a while loop.
2) Used template strings instead of string concat.
3) Consolidating nested if statements.
4) Using tenary expressions if applicable when assigning variables
   to reduce the code overhead.

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


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