sql: don't pass a nil clock in kv_test
sql: don't pass a nil clock in kv_test

The Benchmark tests panic due to `kv_test` passing a `nil` clock to
`rpc.NewContext`. Fix this. See #8081

maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,maxlang/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,veteranlu/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,maxlang/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,veteranlu/cockroach,paperstreet/cockroach