Remove unnecessary os.listdir call and fix infinite loop on Windows if
no file was found in a tree.

Change-Id: I82a8763e807bbc0ce6fcae6b35a370ffe3b34943
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3687234Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Aleksey Khoroshilov <akhoroshilov@brave.com>