when source contains only 1 master, use that as default font

when attempting to run `fontmake -g MyFont.glyphs -o ttf -i` on a file that has only one master (and 1 instance) defined, fontmake fails with this error:

```
fontmake.instantiator.InstantiatorError: Can't generate UFOs from this designspace: no default font.
```

It should simply use that only master source available and treat it as the "default font". The instance(s) will necessarily be on the same location as the only master.

I know one can simply build TTFs from the master, but sometimes you need to apply some custom parameters that are only defined for an instance, and thus you need the `-i` option.