Remove redundant assignment of a variable in function AlterPublicationTables

Hi,

While working on the thread [1]. We encountered that in the
'AlterPublicationTables' function, the assignment 'isnull = true' is
redundant. This assignment is not required, and the variable will be
reassigned before use.
I have attached a patch to address this.

[1]: https://www.postgresql.org/message-id/CALdSSPgiDj8S8POf_6kiUkVrt9-EqYmn%2B2ahDqpdBpyteeAz-Q%40mail.gmail.com

Thanks,
Shlok Kyal