pgsql: Increase test coverage in RI_FKey_pk_upd_check_required()

Increase test coverage in RI_FKey_pk_upd_check_required()

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/

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cdaf4a472776141899dfdb742c9b73581f19f59a

Modified Files
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src/test/regress/expected/foreign_key.out | 24 ++++++++++++++++++++++++
src/test/regress/sql/foreign_key.sql      | 19 +++++++++++++++++++
2 files changed, 43 insertions(+)