Correctness tests fails when there are no events

[SanjayVas]

When there are no events which match filtering predicates in the data set, the correctness test fails for MPC-based R/F measurements. The expected result computed from the unencrypted sketches is always

reach {
  value: 1
}
frequency {
}

This is not considered within the tolerance range compared to an MPC result.

Sample result:

reach {
  value: 6
}
frequency {
  relative_frequency_distribution {
    key: 1
    value: 0.16129032258064516
  }
  relative_frequency_distribution {
    key: 5
    value: 0.44623655913978494
  }
  relative_frequency_distribution {
    key: 7
    value: 0.010752688172043012
  }
  relative_frequency_distribution {
    key: 8
    value: 0.03763440860215054
  }
  relative_frequency_distribution {
    key: 9
    value: 0.3225806451612903
  }
  relative_frequency_distribution {
    key: 10
    value: 0.021505376344086023
  }
}