Use jq to detec spec compliance with sign-exit (#1488)

vc-utils/keymanager.sh

@@ -242,7 +242,7 @@ exit-sign() {
         *) echo "Unexpected return code $__code. Result: $__result"; exit 1;;
     esac
     # This is only reached for 200
-    if [[ ${__result} == '{"data":'* ]]; then
+    if jq -e '.data != null' <<< "${__result}" &>/dev/null; then
         __result=$(echo "${__result}" | jq -c '.data')
     fi