/kind bug

What steps did you take and what happened:
This is half bug report, half question.
I'm looking for a way to automatically apply tolerations to notebooks, pipelines and pods in general which belong to the namespace owned by some user - without the need of the user to select anything in spawner UI form. I'm trying to abstract away taints and tolerations as much as possible.

Since I'm on AWS EKS and PodTolerationRestriction are not available there, I have looked into PodDefaults to achieve this.
PodDefaults seem to accept matchLabels as well as matchExpressions selectors. I have also noticed that KF seems to apply label notebook-name to every new notebook pod - so far so great. I can apply matchExpressions like this:

spec:
  desc: Some poddefault
  selector:
    matchExpressions:
      - { key: notebook-name, operator: Exists}  # matches any notebook
#      - { key: foo, operator: NotIn, values: [ bar ] }  # matches anything in this namespace

This works in general. Any notebook started will receive this default. Or alternatively, when using { key: foo, operator: NotIn, values: [ bar ] } , any pod in this namespace receives the default. It's certainly a hack and I wonder to what degree this is intended behavior.
Also note that users don't have to set anything in the spawner GUI to enable this which is what I intend.

What did you expect to happen:
Now the part that doesn't work smoothly. When omitting matchLabels in PodDefault selector, jupyter-web-app throws an error when entering spawner GUI:

2022-12-06 15:47:23,550 | kubeflow.kubeflow.crud_backend.errors.handlers | ERROR | 'matchLabels'
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/src/apps/common/routes/get.py", line 32, in get_poddefaults
    label = list(pd["spec"]["selector"]["matchLabels"].keys())[0]
KeyError: 'matchLabels'

Looks like the code expects some matchLabels selector. As a result I assume, Configurations drop down menu of spawner doesn't display anything but spawner form can still be submitted.

My question would be if there is any way to apply a PodDefault without this PodDefault showing up in spawner UI? Maybe there is something that could be done with the content of spawner_ui_config.yaml ? Thanks.

Environment:

• Kubeflow version: (version number can be found at the bottom left corner of the Kubeflow dashboard): 1.6

• kfctl version: (use kfctl version): not applicable

• Kubernetes platform: AWS EKS

• Kubernetes version: 1.23

• OS (e.g. from /etc/os-release): Amazon Linux

• Jupyter web app image: docker.io/kubeflownotebookswg/jupyter-web-app:v1.6.1