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/sessionresolver_test.go

@@ -294,7 +294,7 @@ func TestResolverWorksWithProxy(t *testing.T) {
 	<-done
 	// check results
 	if !errors.Is(err, ErrLookupHost) {
-		t.Fatal("not the error we expected")
+		t.Fatal("not the error we expected", err)
 	}
 	if addrs != nil {
 		t.Fatal("expected nil addrs")