fix(apm): only rebuilt cursor if reply is non-null

Author: mbroadst

lib/apm.js

@@ -520,7 +520,7 @@ var Instrumentation = function(core, options, callback) {
                 r = r.documents[0];
               }
 
-              if (commandName.toLowerCase() === 'getmore' && r.cursor == null) {
+              if (commandName.toLowerCase() === 'getmore' && (r == null || r.cursor == null)) {
                 r = {
                   cursor: {
                     id: cursor.cursorState.cursorId,
@@ -533,7 +533,7 @@ var Instrumentation = function(core, options, callback) {
                 (commandName.toLowerCase() === 'find' ||
                   commandName.toLowerCase() === 'aggregate' ||
                   commandName.toLowerCase() === 'listcollections') &&
-                r.cursor == null
+                (r == null || r.cursor == null)
               ) {
                 r = {
                   cursor: {