Allow anonymous user in SMTP notifier

SMTP notifier should be able to send emails with anonymous user, i.e. without
providing username and password in configuration file.

server/src/lib/configuration/Configuration.d.ts

@@ -73,8 +73,8 @@ export interface GmailNotifierConfiguration {
 }
 
 export interface SmtpNotifierConfiguration {
-    username: string;
-    password: string;
+    username?: string;
+    password?: string;
     host: string;
     port: number;
     secure: boolean;

server/src/lib/notifiers/MailSenderBuilder.ts

@@ -28,11 +28,15 @@ export class MailSenderBuilder implements IMailSenderBuilder {
       host: options.host,
       port: options.port,
       secure: options.secure, // upgrade later with STARTTLS
-      auth: {
+    };
+
+    if (options.username && options.password) {
+      smtpOptions.auth = {
         user: options.username,
         pass: options.password
-      }
-    };
+      };
+    }
+
     return new MailSender(smtpOptions, this.nodemailer);
   }
 }

server/test/notifiers/MailSenderBuilder.test.ts

@@ -25,24 +25,41 @@ describe("test MailSenderBuilder", function() {
     Assert.equal(createTransportStub.getCall(0).args[0].auth.pass, "pass_gmail");
   });
 
-  it("should create a smtp mail sender", function() {
-    const mailSenderBuilder = new MailSenderBuilder(Nodemailer);
-    mailSenderBuilder.buildSmtp({
-      host: "mail.example.com",
-      password: "password",
-      port: 25,
-      secure: true,
-      username: "user",
-      sender: "admin@example.com"
+  describe("build smtp mail sender", function() {
+    it("should create a smtp mail sender with authenticated user", function() {
+      const mailSenderBuilder = new MailSenderBuilder(Nodemailer);
+      mailSenderBuilder.buildSmtp({
+        host: "mail.example.com",
+        password: "password",
+        port: 25,
+        secure: true,
+        username: "user",
+        sender: "admin@example.com"
+      });
+      Assert.deepStrictEqual(createTransportStub.getCall(0).args[0], {
+        host: "mail.example.com",
+        auth: {
+          pass: "password",
+          user: "user"
+        },
+        port: 25,
+        secure: true,
+      });
     });
-    Assert.deepStrictEqual(createTransportStub.getCall(0).args[0], {
-      host: "mail.example.com",
-      auth: {
-        pass: "password",
-        user: "user"
-      },
-      port: 25,
-      secure: true,
+
+    it("should create a smtp mail sender with anonymous user", function() {
+      const mailSenderBuilder = new MailSenderBuilder(Nodemailer);
+      mailSenderBuilder.buildSmtp({
+        host: "mail.example.com",
+        port: 25,
+        secure: true,
+        sender: "admin@example.com"
+      });
+      Assert.deepStrictEqual(createTransportStub.getCall(0).args[0], {
+        host: "mail.example.com",
+        port: 25,
+        secure: true,
+      });
     });
   });
 });