Slug error if facet value is number

Collection:

```
---
facets: [year]
---
```

Post:

```
---
year: 2026
---
```

Error:

```
Error: slug() requires a string argument, received number
    at slugify (.../slug/slug.js:120:13)
    at slug (.../slug/slug.js:89:18)
    at createFacetField (.../facet.js:82:62)
```