insights: panic if the insights worker queue has records for an insight series that no longer exists

Created by: coury-clark

When the insights queue worker picks up work to perform searches, it first retrieves the insight metadata. In this case, if the serise no longer exists in the insight_series table, this will return nil and cause a panic.

[enterprise-worker] panic: runtime error: invalid memory address or nil pointer dereference
[enterprise-worker] [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x3743a0f]
[enterprise-worker] goroutine 1118 [running]:
[enterprise-worker] github.com/sourcegraph/sourcegraph/enterprise/internal/insights/background/queryrunner.(*workHandler).Handle(0xc001fcce40, {0x41babf0, 0xc00199f620}, {0x41724e0, 0xc000abbbc0})