[kuwo:category] Fix description and update test

youtube_dl/extractor/kuwo.py

@@ -266,7 +266,6 @@ class KuwoCategoryIE(InfoExtractor):
         'info_dict': {
             'id': '86375',
             'title': '八十年代精选',
-            'description': '这些都是属于八十年代的回忆！',
         },
         'playlist_mincount': 24,
     }
@@ -283,6 +282,8 @@ def _real_extract(self, url):
         category_desc = remove_start(
             get_element_by_id('intro', webpage).strip(),
             '%s简介：' % category_name)
+        if category_desc == '暂无':
+            category_desc = None
 
         jsonm = self._parse_json(self._html_search_regex(
             r'var\s+jsonm\s*=\s*([^;]+);', webpage, 'category songs'), category_id)