[sankaku] fix error when no tags available (closes #259)

[ci skip]

gallery_dl/extractor/sankaku.py

@@ -101,7 +101,7 @@ class SankakuExtractor(SharedConfigMixin, Extractor):
             tags = collections.defaultdict(list)
             tags_html = text.extract(page, '<ul id=tag-sidebar>', '</ul>')[0]
             pattern = re.compile(r'tag-type-([^>]+)><a href="/\?tags=([^"]+)')
-            for tag_type, tag_name in pattern.findall(tags_html):
+            for tag_type, tag_name in pattern.findall(tags_html or ""):
                 tags[tag_type].append(text.unquote(tag_name))
             for key, value in tags.items():
                 data["tags_" + key] = " ".join(value)