variableMeasured not being populated for icephys

e.g. https://dandiarchive.org/dandiset/000109 contains neurodata type VoltageClampSeries. This is supposed to add VoltageClampSeries to the variableMeasured attribute of the assetsSummary ([here](https://github.com/dandi/dandi-cli/blob/d061e3a02f51e0e2067a7ca0cf450b269f2f008e/dandi/tests/test_metadata.py#L456-L460)), however this field is not populated. This problem shows up in many of the icephys datasets, and does not appear to be a problem for ecephys and ophys.

This is a roadblock for me to generate proper figures summarizing dandisets for the DANDI paper