`match' sometimes returns the wrong result

In the data explorer, this query

```
r.expr(["", ""]).map(x => [x.match(''), x.match('X')]);
```

returns

```
[[
  {"end":0,"groups":[],"start":0,"str":""},
  null
], [
  null,
  null
]]
```

The two array that are returned should instead be identical.