The Mail app should not display a blank 'To' if the 'To' is missing

### What Happened?

The Mail app displays a blank "To:" field when the `To:` header is missing.

Here the mail header:
```
Return-path:
Envelope-to: saya@nawan.my.id
Delivery-date: Thu, 02 Jul 2026 11:31:28 +0200
Message-ID:
From: "no_reply@ojk.go.id via RT"
Subject: [NWN #75] [APPK] Pengaduan Anda sudah dicatat dengan nomor [P260700863]
X-RT-Originator: no_reply@ojk.go.id
X-RT-Ticket: rt.nawan.my.id #75
X-Managed-BY: RT 5.0.5+dfsg-2ubuntu0.1~esm1 (http://www.bestpractical.com/rt/)
Reply-To: inbox@nawan.my.id
In-Reply-To:
X-RT-Loop-Prevention: rt.nawan.my.id
Content-Type: multipart/alternative; boundary="----------=_1782984683-4167-0"
References:

Precedence: bulk
Date: Thu, 02 Jul 2026 09:31:23 +0000
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: rt@nawan.my.id
X-AV-Check: Passed
X-System-Sender: rt@nawan.my.id
X-System-UID: 2221
```

### Steps to Reproduce

1. Send email in a way does not have `To:` header
2. Check the email in Mail app and it show blank To field

### Expected Behavior

If `To: is missing from header, do not display blank To:.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

No source file or test is named. Start by reproducing the message in the Mail app and trace how the missing To header is parsed and rendered; done means messages without a To header no longer show an empty To field, while messages with recipients remain unchanged.

Written by the indexing model from the issue text.