Currently, FractionUpdatedDiaObjectsMetricTask raises MetricComputationError if there are 0 pre-existing DIAObjects. However, this is normal in the context of verification, which always starts with an empty PPDB. Therefore, the behavior of this task when there are no DIAObjects to update should be changed to return None (i.e., the metric is not applicable) rather than raising an exception (i.e., the task's calculations failed).