Also allow empty segment in nev.
Also allow empty segment in nev.

When no reset was detected in nev although there was one in nsX, then so
far it raised an erros. But if the first segment of the relevant two can
be determined to be empty, then it is clear all spikes belong to the
next segment

rgerkin/python-neo,JuliaSprenger/python-neo,INM-6/python-neo,NeuralEnsemble/python-neo,apdavison/python-neo,samuelgarcia/python-neo