WASIp3 set-path-and-query normalizes an empty path for `OPTIONS`

[wingo]

Quoth RFC 9110 §4.2.3:

When not being used as the target of an OPTIONS request, an empty path component is equivalent to an absolute path of "/", so the normal form is to provide a path of "/" instead.

However Wasmtime always normalizes "" to "/", even if the method is set to OPTIONS.

Related to WebAssembly/wasi-http#195.