metrics: json: default RUNTIME to 'default'

If a RUNTIME is not set, then save it as 'default' in the JSON
rather than an empty string, as the emptry string does not render
well later in any graphing.

Signed-off-by: Graham Whaley <graham.whaley@intel.com>

metrics/lib/json.bash

@@ -69,7 +69,7 @@ EOF
 
 	local json="$(cat << EOF
 	"test" : {
-		"runtime": "${RUNTIME}",
+		"runtime": "${RUNTIME:-default}",
 		"testname": "${TEST_NAME}"
 	}
 EOF