Set empty locations in log to null

Migration 0033 from 2c69ffb296a5ef27f4beeb8ee81fb6d01e4fc23e
unintentionally set locations that previously had `NULL` values to an
empty `float3d` composite with `NULL` fields. Set these values to `NULL`
again with a migration. Any effects of the empty composites were
mitigated with 626c527332f001649aea0cea14a185e16a50777b, so this change
is for data consistency only.

See acardona/CATMAID#759.