fix(auth-process): only send username/password if provided

NODE-1479

Author: mbroadst

lib/auth_processes/mongodb.js

@@ -53,7 +53,12 @@ class MongoAuthProcess {
     performGssapiCanonicalizeHostName(this.canonicalizeHostName, this.host, err => {
       if (err) return callback(err);
 
-      kerberos.initializeClient(self.service, { user: username, password }, (err, client) => {
+      const initOptions = {};
+      if (password != null) {
+        Object.assign(initOptions, { user: username, password });
+      }
+
+      kerberos.initializeClient(self.service, initOptions, (err, client) => {
         if (err) return callback(err, null);
 
         self.client = client;