url_build() doesn't validate argument

It's quite clear I didn't perfectly read the docs! But it would be quite nice if invalid names are found that an error (and better and informative one!) are provided. 

```r
httr2::url_build(list(host = host, port = port))
#> "//:3000"
```

Whereas I expected: 

```
#> "http://127.0.0.1:3000"
```