Fix potential default mutable issue introduced in #2274

[Avasam]

Introduced in #2274, was missed in the original review. I manually re-reviewed and didn't find any other similar regression.

Simplified demonstration of the issue:

>>> def RegisterServer(catids=[]):
...     catids.append("hi")
...     return catids
...
>>> RegisterServer()
['hi']
>>> RegisterServer()
['hi', 'hi']
>>> RegisterServer()
['hi', 'hi', 'hi']

There are linting rules to avoid using mutable defaults for such reasons. But we don't use them yet.

I'm just reverting this one line as an easy fix

[Avasam]

@mhammond This fixes a potential regression in 307