fix: charge local input tokens when Gemini returns empty response

relay/channel/gemini/relay-gemini.go

@@ -1258,8 +1258,7 @@ func geminiStreamHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http
 	}
 
 	if usage.CompletionTokens <= 0 {
-		str := responseText.String()
-		if len(str) > 0 {
+		if info.ReceivedResponseCount > 0 {
 			usage = service.ResponseText2Usage(c, responseText.String(), info.UpstreamModelName, info.GetEstimatePromptTokens())
 		} else {
 			usage = &dto.Usage{}