fileserver: Fix `file` matcher with empty `try_files` (#4147)

* fileserver: Fix `file` matcher with empty `try_files`

Fixes https://github.com/caddyserver/caddy/issues/4146

If `TryFiles` is empty, we fill it with `r.URL.Path`. In this case, this is `/`. Then later, in `prepareFilePath()`, we run the replacer (which turns `{path}` into `/` at that point) but `file` remains the original value (and the placeholder is still the placeholder there).

So then `strings.HasSuffix(file, "/")` will be `false` for the placeholder, but `true` for the empty `TryFiles` codepath, because `file` was `/` due to being set to the actual request value beforehand.

This means that `suffix` becomes `//` in that case, so after `sanitizedPathJoin`, it becomes `./`, so `strictFileExists`'s `strings.HasSuffix(file, separator)` codepath will return true.

I think we should change the `m.TryFiles == nil` codepath to `m.TryFiles = []string{"{http.request.uri.path}"}` for consistency. (And maybe consider hoisting this to `Provision` cause there's no point doing this on every request). I don't think this "optimization" of directly using `r.URL.Path` is so valuable, cause it causes this edgecase with directories.

* Update modules/caddyhttp/fileserver/matcher.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

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