We might not be correctly clearing OIDC cookies when handling an OIDC callback

This issue has been migrated from [#12782](https://github.com/matrix-org/synapse/issues/12782).

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> Looking at sentry, this one is really weird. It happens when `session == b""`, which might be a result of the cookie clearing done before. Testing it a bit, on Safari, it looks like the cookie clearing line 201 does not clear the cookie properly, and instead sets it to a blank value, hence the issue.

> So, it's a legit exception, and probably a legit bug because we're not clearing the cookie properly.

_Originally posted by @sandhose in https://github.com/matrix-org/synapse/pull/12723#discussion_r874834174_

The lines in question:

https://github.com/matrix-org/synapse/blob/df3c432ed6e5b47f06970251024fd93038a1e6cd/synapse/handlers/oidc.py#L193-L205

And the sentry report: https://sentry.matrix.org/sentry/synapse-matrixorg/issues/219508/?query=is%3Aunresolved

Start in synapse/handlers/oidc.py at the cookie-clearing code around lines 193-205, then review the linked Sentry report and the callback path for the session == b"" case. Reproduce or investigate the Safari behavior described in the issue. Done means the OIDC callback clears the cookie correctly instead of leaving a blank value and avoids the reported exception.

Written by the indexing model from the issue text.