Some code improvements

* Rewrite empty string checks more idiomatically.
* Change strings.ToLower comparisons to strings.EqualFold.
* Rewrite switch statement with only one case as if.

app/proxyman/inbound/inbound.go

@@ -68,7 +68,7 @@ func (m *Manager) GetHandler(ctx context.Context, tag string) (inbound.Handler,
 
 // RemoveHandler implements inbound.Manager.
 func (m *Manager) RemoveHandler(ctx context.Context, tag string) error {
-	if len(tag) == 0 {
+	if tag == "" {
 		return common.ErrNoClue
 	}