Update the startdate column for PMPro members to match their user_registered date if their startdate is empty.

fix_startdates.sql

# If users were imported without a membership_startdate set or
# otherwise don't have a startdate value in the wp_pmpro_memberships_users
# table, this query will set their startdate to the date they registered.
# NOTE: You should change the table names to match your own prefixes.

UPDATE wp_pmpro_memberships_users mu 
  LEFT JOIN wp_users u ON mu.user_id = u.ID 
SET mu.startdate = u.user_registered 
  WHERE mu.startdate IS NULL