fix: support unsetting getPaginationRowModel

[andreasgerstmayr]

Previously, it was not possible to unset the table.options.getPaginationRowModel option, because the result of that function is cached in table._getPaginationRowModel, and therefore the !table._getPaginationRowModel check will always evaluate to false.

Use case: Based on a UI switch, we want to enable or disable the pagination of the React table.