fix: ensure monitor has rtt pinger in when calculating rtt (#2757)

Author: durran

lib/core/sdam/monitor.js

@@ -222,9 +222,10 @@ function checkServer(monitor, callback) {
       }
 
       const isMaster = result.result;
-      const duration = isAwaitable
-        ? monitor[kRTTPinger].roundTripTime
-        : calculateDurationInMs(start);
+      const rttPinger = monitor[kRTTPinger];
+
+      const duration =
+        isAwaitable && rttPinger ? rttPinger.roundTripTime : calculateDurationInMs(start);
 
       monitor.emit(
         'serverHeartbeatSucceeded',