vc_service_request.py: fix check

vc_service_request.py

@@ -100,7 +100,7 @@ def vc_service_request(
     """
     src = prepare_audio(source_audio)
     tgt = prepare_audio(target_audio)
-    if not src or not tgt:
+    if src is None or tgt is None:
         return
     if len(src) >= 60 * 16000 or len(tgt) >= 30 * 16000:
         # input is way too long, dont return anything