rose metadata: fix null first values entry

An empty `rose-app.conf` with a `rose-meta.conf` file that looks like this:
```
[env=METASYN]
values=,foo,bar
value-titles=NOTHING,FOOO,BARRR
```
will give a latent variable in `rose edit` with a value of `foo`.
However, the latent variable value should be the first entry in
`values`, which is a null string.

This bug happens because the parsing to get the first values entry is
a bit crude - once we use the `array_split` function instead, it's much
better and will now handle quotes and escaped commas, etc.