Allow instructions with 0 amplitude

[terliuk]

I think that we need to allow instructions with 0 amplitude rather than rejecting all of them , as it is possible that energy deposit produces 0 electrons or photons , but we still want to keep track about it in truth.

Here is the corresponding line:
https://github.com/XENONnT/WFSim/blob/master/wfsim/strax_interface.py#L617-L618