File type of empty XNC file not detected correctly

When loading the following empty XNC file, gerbv complains that it can't detect the file type and does not load the file.

```
; XNC file generated by gerbonara
M48
METRIC
%
M30
```

AFAICT this is a specification-compliant empty XNC file, and I think gerbv should be able to detect it from the `M48` and `M30` commands.