Allow creating TTL'd sessions without a node

[kyrias]

Description

This is required to be able to hold cross-DC locks.

Testing & Reproduction steps

• Start a cluster with two datacenters, dc1 and dc2, and join a non-server node to dc1.
• Against the client node, execute: xh PUT :8500/v1/session/create dc==dc2 Name=foo TTL=10s 'NodeChecks:=[]' 'Node:=null'

Previously it would fail with an error, with this PR it successfully creates a TTL'd session. After a little bit the new session will correctly disappear from the xh :8500/v1/session/list dc==dc2 output.

Links

Fixes #6828.

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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[kyrias]

Ping?

[svenstaro]

@DanStough could you take a look or suggest a colleague who could?

[github-actions]

This pull request has been automatically flagged for inactivity because it has not been acted upon in the last 60 days. It will be closed if no new activity occurs in the next 30 days. Please feel free to re-open to resurrect the change if you feel this has happened by mistake. Thank you for your contributions.

[svenstaro]

@jmurret could we get a review on this?