extract_raw_waveforms crashes on empty waveform

[Sicheng-package]

When running bc.run_bombcell(..., return_figures=True) on a Kilosort output, bombcell/extract_raw_waveforms.py can index a zero-length waveform and then calls np.max(...), which raises a ValueError：
“zero-size array to reduction operation maximum which has no identity”

[Julie-Fabre]

@m-beau same/related to #348

[m-beau]

See reply at #348