Perhaps ['match'...] shouldn't require 4 arguments

Nobody has claimed this yet.

This wasn't valid as a match expression:

```
'fill-color': ['match', ['get','type'],
/*'park', 'lightgreen',
'wetland','darkgreen',
'school','pink',
'project','red',
'service','cyan',
'polder','hsla(200,70%,50%,0.3)',
'farm', 'hsl(60,90%,60%)',*/
// 'white'
colors.lightgreen
],
```

That is, a `match` where there is only the default value (everything else was temporarily commented out). It seems legitimate by analogy with:

```
switch(x) {
default: f()
}
```

Start by tracing how the map expression parser validates the `match` expression shown in the issue, then look for existing tests covering `match` arguments. Confirm the desired behavior for an expression containing only a default value and add coverage showing whether that form is accepted.

Written by the indexing model from the issue text.