[mod] pylint: minor code change to allow pylint globally

This commit is only a step, it doesn't fix all the issues reported by pylint

searx/engines/qwant.py

@@ -17,7 +17,7 @@ from searx.utils import html_to_text, match_language
 
 
 # engine dependent config
-categories = None
+categories = []
 paging = True
 language_support = True
 supported_languages_url = 'https://qwant.com/region'

searx/engines/xpath.py

@@ -7,6 +7,7 @@ url_xpath = None
 content_xpath = None
 title_xpath = None
 thumbnail_xpath = False
+categories = []
 paging = False
 suggestion_xpath = ''
 results_xpath = ''