fix(sessionresolver): proxy check conditional on existing proxy (#264)

There was a face-palming error in the implementation causing the proxy
check to be implemented also without a proxy.

This meant that we were ALWAYS skipping http3 and system resolvers.

The bug has been introduced in 3.8.0. So, the currently released
version of the probe, sadly, has this beheavior :-(.

Reference issue https://github.com/ooni/probe/issues/1426.

internal/engine/internal/sessionresolver/resolvermaker.go

@@ -97,7 +97,7 @@ func (r *Resolver) newresolver(URL string) (childResolver, error) {
 func (r *Resolver) getresolver(URL string) (childResolver, error) {
 	defer r.mu.Unlock()
 	r.mu.Lock()
-	if re, found := r.res[URL]; found == true {
+	if re, found := r.res[URL]; found {
 		return re, nil // already created
 	}
 	re, err := r.newresolver(URL)