Boolean XPath results are not handled properly

**Problem Statement**

xq doesn't handle XPath boolean results properly, typically used to test a node existence.

**Steps to Reproduce**
```sh
echo -n '<?xml version="1.0"?><root></root>' | xq -x 'boolean(//root)'
```

**Actual Result**
```sh
Error: unknown type error: true
```

**Expected Result**
```sh
true
```