fix config load error when picker is set before the scoerer w/o weight.

[zetxqx]

What type of PR is this?

/kind bug

What this PR does / why we need it:

W/o this PR, the following EPP scheduler config will get nil pointer panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1d12fc9]

apiVersion: inference.networking.x-k8s.io/v1alpha1
    kind: EndpointPickerConfig
    plugins:
    - type: random-picker
    - type: queue-scorer
    - type: kv-cache-utilization-scorer
    - type: prefix-cache-scorer
    schedulingProfiles:
    - name: default
      plugins:
      - pluginRef: random-picker
      - pluginRef: queue-scorer
      - pluginRef: kv-cache-utilization-scorer
      - pluginRef: prefix-cache-scorer

We should not assume picker is always the last plugin. Thus, we should not break earlier before setting the defaults for all scorers.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

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[LukeAVanDrie]

You may be interested in #1953. I tried to harden this path broadly. Your change will likely be merged first as it is well-scoped. I'll make sure to verify that this is also fixed in my refactoring.

[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, zetxqx

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