handle empty response for gemini

pkg/llm/llm.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"reflect"
 	"strconv"
+	"strings"
 	"sync"
 	"time"
 
@@ -407,6 +408,9 @@ func (c *cached) String(ctx context.Context, messages []string) (string, error)
 	if err != nil {
 		return "", err
 	}
+	if strings.Trim(value, " \n\r\t") == "" {
+		return "", errors.New("empty response") // Gemini may occur this.
+	}
 
 	// TODO: reduce copies.
 	if err = c.kvStorage.Set(ctx, []byte(keyStr), []byte(value), 65*time.Minute); err != nil {