fix(translation): 修复处理中间结果时的空指针异常

避免在translationHistory为空时访问last属性

lib/modules/translation/controllers/translation_controller.dart

@@ -677,8 +677,8 @@ class TranslationController extends GetxController with WidgetsBindingObserver {
   void handleIntermediateResult(String text) {
     if (text.isEmpty) return;
     print(
-        "handleIntermediateResult:${translationHistory.isEmpty}${translationHistory.last.isIntermediate}");
-    if (translationHistory.isEmpty || !translationHistory.last.isIntermediate) {
+        "handleIntermediateResult:${translationHistory.isEmpty}${translationHistory.isNotEmpty ? translationHistory.last.isIntermediate : 'N/A'}");
+    if (translationHistory.isEmpty || (translationHistory.isNotEmpty && !translationHistory.last.isIntermediate)) {
       currentSessionId ??= DateTime.now().millisecondsSinceEpoch.toString();
 
       final newItem = TranslationItem(