FIX: Digest frequency issue on user creation

If `default email digest frequency` was set to "Never", users would get
a `digest_after_minutes` set to `nil` which triggered this error
in the logs if/when the site eventually changed that setting and
enabled digests:

```
NoMethodError (undefined method `>=' for nil:NilClass)
/var/www/discourse/app/mailers/user_notifications.rb:227:in `digest'
```

spec/models/user_option_spec.rb

@@ -33,6 +33,20 @@ describe UserOption do
     it "should not hide the profile and presence by default" do
       expect(user.user_option.hide_profile_and_presence).to eq(false)
     end
+
+    it "should correctly set digest frequency" do
+      SiteSetting.default_email_digest_frequency = 1440
+      user = Fabricate(:user)
+      expect(user.user_option.email_digests).to eq(true)
+      expect(user.user_option.digest_after_minutes).to eq(1440)
+    end
+
+    it "should correctly set digest frequency when disabled" do
+      SiteSetting.default_email_digest_frequency = 0
+      user = Fabricate(:user)
+      expect(user.user_option.email_digests).to eq(false)
+      expect(user.user_option.digest_after_minutes).to eq(0)
+    end
   end
 
   describe "site settings" do