Werk #14343: KUBE agent_kube: change ContainerTerminatedState's start and end time to be optional

Previously, the Kubernetes special agent raised the error [special_kube] TypeError: Can not convert to timestamp: 'None' of type <class 'NoneType’> when a terminated container had no information about either its start or finish time. With this werk, the assumption that the start and finish times are always set is removed, and is thus consistent with the Kubernetes specification. The check plug-in is also modified to display the time-related information depending on what fields are returned.

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