Agent: "spec.PolicyID is empty" warning is raised despite Agent is running in standalone mode

[barkbay]

The Agent webhook attempts to check if PolicyID is set whether or not the Agent is running in fleet or standalone mode:

func (a *Agent) GetWarnings() []string {
	if a == nil {
		return nil
	}
	if len(a.Spec.PolicyID) == 0 {
		return []string{fmt.Sprintf("%s %s/%s: %s", Kind, a.Namespace, a.Name, MissingPolicyIDMessage)}
	}
	return nil
}

This check should only be done in fleet mode.