Werk #13252: netapp_api_vs_traffic services become PENDING if time correspondent does not change

netapp_api_vs_traffic used IO operation counter instead of time at some places which leads to PENDING services if those values do not change over time leading to a non-computable IO rate (and thus raising an exception). This change makes netapp_api_vs_traffic handle that case returning '-' instead of a rate (but not becoming PENDING any more).

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