osxkeychain: list: do not error out when keychain is empty

Commit 4cdcdc2 replaced the in-tree Objective-C code with github.com/keybase/go-keychain
and inadvertently introduced a new failure mode on the `List` operation -
it now fails when the keychain is empty.

Before:

```
$ ./bin/build/docker-credential-osxkeychain list
{}
```

After:

```
$ ./bin/build/docker-credential-osxkeychain list
credentials not found in native keychain
```

Signed-off-by: Albin Kerouanton <[email protected]>

Author: akerouanton

osxkeychain/osxkeychain.go

@@ -117,8 +117,6 @@ func (h Osxkeychain) List() (map[string]string, error) {
 		default:
 			return nil, err
 		}
-	} else if len(res) == 0 {
-		return nil, credentials.NewErrCredentialsNotFound()
 	}
 
 	resp := make(map[string]string)