fix(TrafficRoute): use default value when choiceCount is 0

[lukidzi]

Checklist prior to review

Api says that when choiceCount is not defined we are using a default value of 2 which is correct from the envoy perspective but not from kuma API.

GetLeastRequest() returns an object that has choiceCount equal 0, so when a user creates a policy without choiceCount it doesn't fail on the API but will fail on xds configuration generation because envoy requires a minimum value of 2.
I've changed a bit how validation works now:

• validate object if the value is equal 1, that won't cause xds configuration to fail and break the sidecar
• when the value is equal to 0 we assume that it's an empty object and we set it to default 2

• Link to relevant issue as well as docs and UI issues -- fix: TrafficRoute doesn't set LB type to LeastRequest when choiceCount is not specified #7932
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label) --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?

[jakubdyszkiewicz]

Is it fixed in MeshLoadBalancingStrategy already?

[slonka]

If we can't detect 0 vs no value like in new policies let's do this.

[lukidzi]

Is it fixed in MeshLoadBalancingStrategy already?

Yes, kubevalidator validates if the value is bigger than or equal to 2, also when an empty body it treats it as a null so it works.