Pod creation fails in certain cases when generateUniqueAppId is enabled

As part of YUNIKORN-1273 new functionality was added to generate unique application ids.

A unique appId is generated based on the podUid and the namespace name. We used podUID instead of other options like timestamp and random hash, as regenerating the appId with the podUID during the recovery would generate the same appId.

But in certain cases, when the request reaches the admission controller, the podUID is not available and the request would fail due to an invalid label value.

Example pod that fails with this error:

apiVersion: v1
kind: Pod
metadata:
  name: sleepy
spec:
  containers:
  - name: sleepy
    image: sleepy:0.1.0

kubectl apply -f sleepypod.yaml

Error:

The Pod "sleepy" is invalid: metadata.labels: Invalid value: "default-": a valid label must be an empty string or consist of alphanumeric characters, '-', '_' or '.', and must start and end with an alphanumeric character (e.g. 'MyValue',  or 'my_value',  or '12345', regex used for validation is '(([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?')