Jetpack Sync: PHP notice for checksum process

> there’s a big spike in these notices from Jetpack today: “Undefined index: shared_salt”

https://github.com/Automattic/jetpack/blob/e60446527adaedf8ad10ee43456a068669c2bc87/projects/plugins/jetpack/json-endpoints/jetpack/class.jetpack-json-api-sync-endpoint.php#L116

This happens whether the site runs 9.5 or 9.6.

Reported here:
https://wordpress.org/support/topic/spike-in-these-all-of-a-sudden-undefined-index-shared_salt/

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@bisko @mdbitz Do you think you could take a look?

Start at projects/plugins/jetpack/json-endpoints/jetpack/class.jetpack-json-api-sync-endpoint.php around line 116 and trace how shared_salt is accessed during checksum processing. Compare the reported behavior on Jetpack 9.5 and 9.6; done means the sync endpoint no longer emits the Undefined index: shared_salt notice.

Written by the indexing model from the issue text.