Default first_name/last_name to empty string in profile import

Exporters omit name fields from profiles when they are blank, so
prof[:first_name] and prof[:last_name] can be nil. Passing nil directly
to assign_form_response_attributes triggers a NOT NULL violation on the
user_con_profiles table. .to_s converts nil to "" safely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nbudin

app/services/import_convention_data_service.rb

@@ -283,8 +283,8 @@ def import_user_con_profiles(convention, user_map)
 
   def profile_form_response_attrs(prof)
     {
-      first_name: prof[:first_name],
-      last_name: prof[:last_name],
+      first_name: prof[:first_name].to_s,
+      last_name: prof[:last_name].to_s,
       nickname: prof[:nickname],
       bio: prof[:bio],
       show_nickname_in_bio: prof[:show_nickname_in_bio],