refactor(sdam): revert null => undefined changes until 4.x

Author: mbroadst

lib/core/sdam/topology.js

@@ -423,7 +423,7 @@ class Topology extends EventEmitter {
     const transaction = session && session.transaction;
 
     if (isSharded && transaction && transaction.server) {
-      callback(undefined, transaction.server);
+      callback(null, transaction.server);
       return;
     }
 
@@ -444,7 +444,7 @@ class Topology extends EventEmitter {
           transaction.pinServer(selectedServer);
         }
 
-        callback(undefined, selectedServer);
+        callback(null, selectedServer);
       }
     );
   }
@@ -567,11 +567,11 @@ class Topology extends EventEmitter {
 
   auth(credentials, callback) {
     if (typeof credentials === 'function') (callback = credentials), (credentials = null);
-    if (typeof callback === 'function') callback(undefined, true);
+    if (typeof callback === 'function') callback(null, true);
   }
 
   logout(callback) {
-    if (typeof callback === 'function') callback(undefined, true);
+    if (typeof callback === 'function') callback(null, true);
   }
 
   // Basic operation support. Eventually this should be moved into command construction
@@ -664,7 +664,7 @@ class Topology extends EventEmitter {
         isWriteCommand(cmd);
 
       const cb = (err, result) => {
-        if (!err) return callback(undefined, result);
+        if (!err) return callback(null, result);
         if (!isRetryableError(err)) {
           return callback(err);
         }