Skeletal element category cannot be updated to empty in sample archaeology update form

Suppose we want to update a sample via the sample archaeology update form (https://screening.reichdna.hms.harvard.edu/samples/sample_archaeology_update). If the sample begins with a skeletal element category assigned and we want to switch it to null/empty/nothing, the form currently cannot do so. This seems to be because of these lines in samples/intake.py:

if len(row.skeletal_element_category) > 0:
    sample.skeletal_element_category = SkeletalElementCategory.objects.get(category__iexact=row.skeletal_element_category)

I.e. the sample's skeletal element category is only updated if we are trying to update it to something non-null/non-empty, making it impossible to update the category to null/empty.

Start in samples/intake.py at the skeletal_element_category update logic described in the issue, then trace how the sample archaeology update form passes an empty value. Verify the form can change an existing category to empty or null and that the sample remains cleared after the update.