Allow `export_report` to run without waveforms

[chrishalcrow]

Small edit to export_report so that it will generate a report without waveforms.

When we're being speedy, we often don't compute or don't copy over waveforms. This PR just changes export_report so that it does/doesn't include the UnitWaveformDensityMapWidget if the sorting_analyzer does/doesn't have waveforms computed.

A unit report without waveforms:

computed using

import spikeinterface.full as si
rec, sort = si.generate_ground_truth_recording()
sa = si.create_sorting_analyzer(recording=rec, sorting=sort)
sa.compute(["random_spikes",  "templates", "spike_amplitudes", "correlograms", "template_similarity"])
si.export_report(sorting_analyzer=sa, output_folder="my_report")