civicrm/admin/reltype - Fix warning

civicrm/admin/reltype - Fix warning

Scenario:
 1. Create a new relationship-type. It should target "All Contacts" (not "Individual" or "Organization"). (Note: In the schema, "All Contacts" translates to `contact_type_a=NULL`.)
 2. Reload the list of relationship-types
 3. Observe: Warnings

Notes:
 1. The relationship type appears to work correctly if you try to create
    relationship-instances, so this feels like a cosmetic issue on the listing
    screen.
 2. What a silly place for that code live. Keep calm and carry on.