pgsql: Fix dblink_build_sql_insert() and related functions to handle

Log Message:
-----------
Fix dblink_build_sql_insert() and related functions to handle dropped
columns correctly.  In passing, get rid of some dead logic in the
underlying get_sql_insert() etc functions --- there is no caller that
will pass null value-arrays to them.

Per bug report from Robert Voinea.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.38.4.9 -> r1.38.4.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.38.4.9&r2=1.38.4.10)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.13.4.2 -> r1.13.4.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.13.4.2&r2=1.13.4.3)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.12.4.1 -> r1.12.4.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql?r1=1.12.4.1&r2=1.12.4.2)