Removed some pandas warnings

[micheles]

Part of #9020. The warning in openquake/commonlib/tests/reinsurance_test.py was

  /home/michele/oq-engine/openquake/risklib/reinsurance.py:64: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
    [indices] = np.where(dframe.liability.to_numpy() == '')

and in test_ideductible_exposure was

  <__array_function__ internals>:180: DeprecationWarning: Calling nonzero on 0d arrays is deprecated, as it behaves surprisingly. Use `atleast_1d(cond).nonzero()` if the old behavior was intended. If the context of this warning is of the form `arr[nonzero(cond)]`, just use `arr[cond]`.

Also improved a reinsurance test to consider more events.