Fix vault pillar rendering as all or nothing for each minion

[MKLeb]

What does this PR do?

Ensures that pillar rendering doesn't fail if all secrets aren't available to each minion.

The session cache was holding the token under certain circumstances after failing to retrieve a secret. The token was then invalid, but it was not being removed from cache, thus resulting in permission denied errors.

What issues does this PR fix or reference?

Fixes: #61821

Previous Behavior

The pillar would fail to render after encountering the first secret a minion was denied access to.

New Behavior

Now the pillar renders properly.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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