Throw ERROR when publish_generated_columns is specified without a value.

Throw ERROR when publish_generated_columns is specified without a value.

Previously, specifying the publication option 'publish_generated_columns'
without an explicit value would incorrectly default to 'stored', which is
not the intended behavior.

This patch fixes the issue by raising an ERROR when no value is provided
for 'publish_generated_columns', ensuring that users must explicitly
specify a valid option.

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Backpatch-through: 18, where it was introduced
Discussion: https://postgr.es/m/CAHut+PsCUCWiEKmB10DxhoPfXbF6jw5RD9ib2LuaQeA_XraW7w@mail.gmail.com