Use a default empty set for usernames

[ndrwrbgs]

Use a default empty set for `usernames`

Fixes

```
Traceback (most recent call last):
  File "/Users/andrewa/stripe/Uscrapper/Uscrapper.py", line 138, in <module>
    extract_details(args.url, args.generate_report, args.nonstrict)
  File "/Users/andrewa/stripe/Uscrapper/Uscrapper.py", line 68, in extract_details
    if usernames:
UnboundLocalError: local variable 'usernames' referenced before assignment
```

on command `python Uscrapper.py  -u https://github.com/z0m31en7/Uscrapper/tree/main -O`

```
$ python -V
Python 3.9.11
```