autoupdate.ps1: Fixes bug for single line hash extraction (#3015)
autoupdate.ps1: Fixes bug for single line hash extraction (#3015)

Fix a bug introduced in #2996 that single line hash extraction is broken.

When `find_hash_in_textfile()` is called in `get_hash_for_app()`, `$null` is used as `$regex` value for default single line hash extraction and the default value (`'^([a-fA-F0-9]+)$'`) is overriden, which leads a error of "hash not found".
coonce/scoop,kodybrown/scoop,lukesampson/scoop,rasa/scoop,nikolasd/scoop,berwyn/scoop