fix(ur): actually send usage report directly when enabled (#9736)

There was a bug that the unique ID was not set when reporting was
enabled, and thus the reports where rejected by the server. The unique
ID got set only on startup, so next time Syncthing restarted.

This makes sure to set the unique ID when blank.

lib/config/config_test.go

@@ -305,6 +305,11 @@ func TestOverriddenValues(t *testing.T) {
 		t.Error(err)
 	}
 
+	if cfg.Options().URUniqueID == "" {
+		t.Error("expected usage reporting unique ID to be set since usage reporting is enabled")
+	}
+	expected.URUniqueID = cfg.Options().URUniqueID // it's random
+
 	if diff, equal := messagediff.PrettyDiff(expected, cfg.Options()); !equal {
 		t.Errorf("Overridden config differs. Diff:\n%s", diff)
 	}