This checks the case where the primary key has at least one null
column.
Reviewed-by: Alvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Mi Tar <mmitar@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/7ae17c95-0c99-d420-032a-c271f510112b@2ndquadrant.com/