Reference implementation: where clause does not validate boolean return type

Moved from HL7/sql-on-fhir#310.

The `validate.json > where with path resolving to not boolean` test is failing in the sof-js reference implementation.

## Test case

The test expects an error when a `where` clause path resolves to a non-boolean value:

```json
{
"title": "where with path resolving to not boolean",
"tags": ["shareable"],
"view": {
"resource": "Patient",
"status": "active",
"select": [
{
"column": [
{
"name": "id",
"path": "id",
"type": "id"
}
]
}
],
"where": [
{
"path": "name.family"
}
]
},
"expectError": true
}
```

## Problem

The test expects the view runner to throw an error because `name.family` evaluates to a string (e.g., "F1.1"), not a boolean. However, the sof-js reference implementation does not throw an error.

In `sof-js/src/index.js`, the `select` function has this check:

```javascript
assert(val === undefined || typeof val === 'boolean', "'where' expression path should return 'boolean'")
return val
```

When `name.family` returns a string like "F1.1", JavaScript's truthiness evaluation causes the assertion to fail silently or the value is coerced to truthy, rather than throwing an error.

## Expected behaviour

The view runner should throw an error when a `where` clause path evaluates to a non-boolean value (string, number, etc.).

## Actual behaviour

The view runner does not throw an error and the test fails.

Start with the select function and its where-expression check in sof-js/src/index.js, then inspect the `validate.json > where with path resolving to not boolean` conformance case. Run the reference implementation's validation tests and confirm that a non-boolean path result produces an error while boolean and undefined results retain their expected behavior.

Written by the indexing model from the issue text.