beet update: suppress "None ->" changes

Hello,

I wonder why, when doing a "beet update" to re-scan my directory, I got a display about all the albums, but some got no additional information and the others do, and information displayed is not always the same.

I got something like:

```
Radiohead - Hail to the Thief. (The Gloaming.) - A Wolf at the Door. (It Girl. Rag Doll.)
asin: None ->
disctitle: None ->
bitrate: 962026 -> 1022076
catalognum: None ->
media: None ->
Refused - The Shape of Punk to Come - Tannhäuser / Derivè
asin: None ->
disctitle: None ->
bitrate: 842050 -> 845239
Refused - Songs to Fan the Flames of Discontent - Return to the Closet
Refused - Songs to Fan the Flames of Discontent - It's Not O.K...
Raspigaous - Chiens des quais - 1999
disctitle: None ->
catalognum: None ->
Raspigaous - Chiens des quais - Le Panier
disctitle: None ->
catalognum: None ->
```

I don't understand why.