tsv output should not contain empty value for threshold and holding current

it happens that the mecombo_emodel.tsv contains missing entries for threshold or holding current, leading to rows with 7 entries instead of 8 for instance.
This is incompatible with what the simulator expects (it expects 8 values to be provided).

Probably it relates to that line:
https://github.com/BlueBrain/BluePyMM/blob/1f87b5125d178a0a6d65672e612fefbca38ae65d/bluepymm/select_combos/megate_output.py#L94

We may consider to discard the cells from the output for instance.