multiTestContext: fix NPE in SendNext
multiTestContext: fix NPE in SendNext

Closes #8406.

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