Test for nil on expire expanded postings

[alanprot]

What this PR does:
Adding a test case for #6518

=== RUN   TestIngesterDeletionRace
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0xf0 pc=0x1016cba4c]

goroutine 177 [running]:
github.com/cortexproject/cortex/pkg/ingester.(*Ingester).expirePostingsCache(0x14000d4d908, {0x102160e90, 0x1400004d9f0})
	/Users/approtas/workspace/cortex/pkg/ingester/ingester.go:2814 +0x3c
github.com/cortexproject/cortex/pkg/ingester.(*Ingester).starting.NewTimerService.func2({0x102160e90, 0x1400004d9f0})
	/Users/approtas/workspace/cortex/pkg/util/services/services.go:33 +0xd0
github.com/cortexproject/cortex/pkg/util/services.(*BasicService).main(0x14000ae41e0)
	/Users/approtas/workspace/cortex/pkg/util/services/basic_service.go:190 +0x170
created by github.com/cortexproject/cortex/pkg/util/services.(*BasicService).StartAsync.func1 in goroutine 154
	/Users/approtas/workspace/cortex/pkg/util/services/basic_service.go:119 +0xfc

Which issue(s) this PR fixes:
Fixes #

Checklist

• [NA] Tests updated
• [NA] Documentation added
• [NA] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]