refactor: don't use null for missing callback values

lib/core/connection/connect.js

@@ -114,18 +114,18 @@ function performInitialHandshake(conn, options, _callback) {
   const start = new Date().getTime();
   runCommand(conn, 'admin.$cmd', handshakeDoc, options, (err, ismaster) => {
     if (err) {
-      callback(err, null);
+      callback(err);
       return;
     }
 
     if (ismaster.ok === 0) {
-      callback(new MongoError(ismaster), null);
+      callback(new MongoError(ismaster));
       return;
     }
 
     const supportedServerErr = checkSupportedServer(ismaster, options);
     if (supportedServerErr) {
-      callback(supportedServerErr, null);
+      callback(supportedServerErr);
       return;
     }
 
@@ -159,7 +159,7 @@ function performInitialHandshake(conn, options, _callback) {
       return;
     }
 
-    callback(null, conn);
+    callback(undefined, conn);
   });
 }
 
@@ -350,7 +350,7 @@ function runCommand(conn, ns, command, options, callback) {
     // ignore all future errors
     conn.on('error', noop);
 
-    _callback(err, null);
+    _callback(err);
   }
 
   function messageHandler(msg) {
@@ -363,7 +363,7 @@ function runCommand(conn, ns, command, options, callback) {
     conn.removeListener('message', messageHandler);
 
     msg.parse({ promoteValues: true });
-    _callback(null, msg.documents[0]);
+    _callback(undefined, msg.documents[0]);
   }
 
   conn.setSocketTimeout(socketTimeout);
@@ -382,7 +382,7 @@ function authenticate(conn, credentials, callback) {
   const provider = AUTH_PROVIDERS[mechanism];
   provider.auth(runCommand, [conn], credentials, err => {
     if (err) return callback(err);
-    callback(null, conn);
+    callback(undefined, conn);
   });
 }