Clear 'xid' in dummy async notify entries written to fill up pagesHEADmaster

Before we started to freeze async notify entries (commit 8eeb4a0f7c),
no one looked at the 'xid' on an entry with invalid 'dboid'. But now
we might actually need to freeze it later. Initialize them with
InvalidTransactionId to begin with, to avoid that work later.

Álvaro pointed this out in review of commit 8eeb4a0f7c, but I forgot
to include this change there.

Author: Álvaro Herrera <[email protected]>
Discussion: https://www.postgresql.org/message-id/[email protected]
Backpatch-through: 14

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