release-22.1: sql/gcjob: deal with a missing system config span

[ajwerner]

Backport 1/1 commits from #83280.

/cc @cockroachdb/release

---

This can cause panics if not handled. It must be quite rare that the jobs
subsystem gets going before we have a system config, but clearly it's
possible.

The contract on the SystemConfigProvider is that it can return nil. This case has to be
handled.

cockroach/pkg/config/provider.go

Lines 16 to 17 in 8878715

	// GetSystemConfig returns the local unmarshaled version of the system
	// config. Returns nil if the system config hasn't been set yet.

See https://teamcityartifacts.crdb.io/presignedTokenAuth/158ce0ab-b5b6-4cc1-9f1b-672d20d0a56e/repository/download/Cockroach_Ci_Tests_LocalRoachtest/5545173:id/acceptance/version-upgrade/run_1/artifacts.zip!/logs/2.unredacted/cockroach-stderr.05b0f904debc.roach.2022-06-23T14_53_26Z.103197.log

Release note (bug fix): Fixed a bug where a panic could occur during server
startup when restarting a node which is running a GC job.

Release justification: Fixes a bug which can cause a node to crash during startup.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[ajwerner]

TFTR!