Fix the robot email script

Fix the robot email script

```
Traceback (most recent call last):
  File "~/linux-firmware-robot/./contrib/process_linux_firmware.py", line 339, in <module>
    process_database(conn, remote)
  File "~/linux-firmware-robot/./contrib/process_linux_firmware.py", line 276, in process_database
    reply_email(mbox, branch)
  File "~/linux-firmware-robot/./contrib/process_linux_firmware.py", line 86, in reply_email
    reply["To"] += email.utils.formataddr(target)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'
```

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>