fix: PC-12544 Add validation for components list to be required (even if empty)

[marcinlawnik]

Motivation

New SLO has elements at depth of 4:

objectives:
  composite:
    components:
      objectives:

A list of objectives can be empty.

It may be possible that a typo is made at the highest depth (objectives), which results in no error, but the list being empty (while the user expects the objectives to be there).

Summary

Made the components list required, so that an empty array(YAML) / slice(Go) needs to be passed instead of omitted YAML property or nil slice.

Related changes

None

Testing

Added unit tests, sample YAML with the issue:

- apiVersion: n9/v1alpha
  kind: SLO
  metadata:
    name: slo-test
    project: sample-project
  spec:
    alertPolicies: []
    budgetingMethod: Occurrences
    objectives:
    - displayName: Test
      name: objective-1
      composite:
        maxDelay: 20m
        objectives: # missing components parent`
          - project: splunk-direct
            slo: splunk-direct-counts-calendar
            objective: objective-1
            weight: 1
            whenDelayed: CountAsGood
          - project: splunk-direct
            slo: splunk-direct-raw-calendar
            objective: objective-3
            weight: 1
            whenDelayed: CountAsGood
          - project: thousandeyes
            slo: net-loss-rolling-timeslices
            objective: objective-1
            weight: 1
            whenDelayed: CountAsGood
      target: 0.9
    service: service
    timeWindows:
    - count: 1
      isRolling: true
      unit: Day
  status:
    updatedAt: "2022-01-21T11:22:14Z"

Correct YAML:

- apiVersion: n9/v1alpha
  kind: SLO
  metadata:
    name: slo-test
    project: sample-project
  spec:
    alertPolicies: []
    budgetingMethod: Occurrences
    objectives:
    - displayName: Test
      name: objective-1
      composite:
        maxDelay: 20m
        components:
          objectives:
            - project: splunk-direct
              slo: splunk-direct-counts-calendar
              objective: objective-1
              weight: 1
              whenDelayed: CountAsGood
            - project: splunk-direct
              slo: splunk-direct-raw-calendar
              objective: objective-3
              weight: 1
              whenDelayed: CountAsGood
            - project: thousandeyes
              slo: net-loss-rolling-timeslices
              objective: objective-1
              weight: 1
              whenDelayed: CountAsGood
      target: 0.9
    service: service
    timeWindows:
    - count: 1
      isRolling: true
      unit: Day
  status:
    updatedAt: "2022-01-21T11:22:14Z"