Fix specifying empty list in provider and model allow/denylists

[MaicoTimmerman]

When setting c.AiExtension.allowed_providers = [], it acts like the default value None and will show all models.

We actually meant to not show any models at all. This is useful for when dynamically selecting models based on user/deployment.

[srdas]

Related to Issue #913
Upstream issue in traitlets : ipython/traitlets#908
I tested the PR and it does not resolve the above issue.

[dlqqq]

To clarify, #913 documents a known issue where a user cannot specify a list containing a single item for a list configurable via traitlets. This was a regression introduced by traitlets==5.13.0, and @srdas verified that this regression continues to exist in the latest version of traitlets. We're exploring ways to remove our dependency on traitlets in v3.

However, specifying '--AiExtension.{allowed,blocked}_providers=[]' to jupyter-lab works, and does set the trait's value to an empty list in the backend. This can be verified from the server logs on init:

% jupyter lab '--AiExtension.allowed_providers=[]'
[I 2025-01-06 09:44:56.529 ServerApp] jupyter_ai | extension was successfully linked.
...
[I 2025-01-06 09:44:56.562 AiExtension] Configured provider allowlist: []

The code looks good to me. I'll approve and start the CI. @srdas You can help verify this PR if you have extra time.

[srdas]

The code looks good to me. I'll approve and start the CI. @srdas You can help verify this PR if you have extra time.

Confirmed this works as intended using the approach above:

[dlqqq]

@srdas Thank you for verifying! Looks like the unit tests failed, so I've updated them to pass and assert this PR's new changes.

[dlqqq]

@meeseeksdev please backport to 2.x