Prevent panicking during shutdown when SQS consumer is disabled

[lcharkiewicz]

Fixing a panic when SQS consumer is disabled and auth is closing the Audit Logger.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x55659d2f4e1a]

goroutine 6295 [running]:
github.com/gravitational/teleport/lib/events/athena.(*Log).Close(0x0?)
        github.com/gravitational/teleport/lib/events/athena/athena.go:546 +0x1a
github.com/gravitational/teleport/lib/events.(*AuditLog).Close(0xc0015f8c00)
        github.com/gravitational/teleport/lib/events/auditlog.go:1075 +0x59
github.com/gravitational/teleport/lib/auth.(*Server).Close(0xc0015b2c08)
        github.com/gravitational/teleport/lib/auth/auth.go:1849 +0x72
github.com/gravitational/teleport/lib/service.(*TeleportProcess).StartShutdown.func1()
        github.com/gravitational/teleport/lib/service/service.go:6012 +0x17f
created by github.com/gravitational/teleport/lib/service.(*TeleportProcess).StartShutdown in goroutine 1
        github.com/gravitational/teleport/lib/service/service.go:6004 +0x2d8

Disabling consumer was added in #46770.

Changelog: Prevent panicking during shutdown when SQS consumer is disabled

[public-teleport-github-review-bot]

@lcharkiewicz See the table below for backport results.

Branch	Result
branch/v15	Create PR
branch/v16	Create PR
branch/v17	Create PR