Fix crash because mailboxName is null

mailboxName is always a non-empty string. even for apps that don't use
it, we allocate a mailbox with .app suffix

migrations/20190303024631-apps-default-mailboxName.js

@@ -0,0 +1,27 @@
+'use strict';
+
+var async = require('async'),
+    crypto = require('crypto'),
+    fs = require('fs'),
+    os = require('os'),
+    path = require('path'),
+    safe = require('safetydance'),
+    tldjs = require('tldjs');
+
+exports.up = function(db, callback) {
+    db.all('SELECT * FROM apps, subdomains WHERE apps.id=subdomains.appId AND type="primary"', function (error, apps) {
+        if (error) return callback(error);
+
+        async.eachSeries(apps, function (app, iteratorDone) {
+            if (app.mailboxName) return iteratorDone();
+
+            const mailboxName = (app.subdomain ? app.subdomain : JSON.parse(app.manifestJson).title.toLowerCase().replace(/[^a-zA-Z0-9]/g, '')) + '.app';
+
+            db.runSql('UPDATE apps SET mailboxName=? WHERE id=?', [ mailboxName, app.id ], iteratorDone);
+        }, callback);
+    });
+};
+
+exports.down = function(db, callback) {
+    callback();
+};