Setting scan span properties can panic

When From/To are nil then we can see something like

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x14db732]

goroutine 141 [running]:
github.com/couchbase/gocb/v2.(*Collection).newRangeScanOpManager(0xc0001d9600, {0x18d7120?, 0xc000347cd0?}, 0x400, {0x18df340?, 0xc000271560}, {0x0?, 0x0?}, 0xc000503260, 0x0, ...)
    /Users/charlesdixon/dev/go-workspace/pkg/mod/github.com/couchbase/gocb/v2@v2.6.1-0.20221206180857-3fecd20bede9/rangescanopmanager.go:350 +0x452
github.com/couchbase/gocb/v2.(*Collection).Scan(0xc0001d9600, {0x18d7120, 0xc000347cd0}, 0x16?)
    /Users/charlesdixon/dev/go-workspace/pkg/mod/github.com/couchbase/gocb/v2@v2.6.1-0.20221206180857-3fecd20bede9/collection_rangescan.go:177 +0x1bf
github.com/couchbaselabs/transactions-fit-performer/sdk.(*Executor).performOperation(0xc00009da10, 0x1793d20?, {0x18d6740?, 0xc000497f80?})
    /Users/charlesdixon/dev/transactions-fit-performer/performers/go/sdk/executor.go:297 +0x123f
github.com/couchbaselabs/transactions-fit-performer/sdk.(*Executor).PerformOperation(0xc0002f9c00?, 0x17c28e0?, {0x18d6740, 0xc000497f80})
    /Users/charlesdixon/dev/transactions-fit-performer/performers/go/sdk/executor.go:84 +0x27
main.(*HorizontalScaleRunner).runSDKWorkload(0xc000508000, 0xc0001325d0)
    /Users/charlesdixon/dev/transactions-fit-performer/performers/go/horizontalScaleRunner.go:110 +0xce
main.(*HorizontalScaleRunner).Run(0xc000508000)
    /Users/charlesdixon/dev/transactions-fit-performer/performers/go/horizontalScaleRunner.go:76 +0x166
created by main.(*Performer).Run
    /Users/charlesdixon/dev/transactions-fit-performer/performers/go/performer.go:268 +0x86e
exit status 2