infopage: a .md file can be remove without crash

An administrator might decide to remove some of the .md files.
This commit make sure to not crash the application.

searx/infopage/__init__.py

@@ -157,10 +157,9 @@ class InfoPageSet:  # pylint: disable=too-few-public-methods
             return None
 
         cache_key = (pagename, locale)
-        page = self.CACHE.get(cache_key)
 
-        if page is not None:
-            return page
+        if cache_key in self.CACHE:
+            return self.CACHE[cache_key]
 
         # not yet instantiated
 
@@ -183,4 +182,6 @@ class InfoPageSet:  # pylint: disable=too-few-public-methods
             if fallback_to_default and page is None:
                 page_locale = self.locale_default
                 page = self.get_page(page_name, self.locale_default)
-            yield page_name, page_locale, page
+            if page is not None:
+                # page is None if the page was deleted by the administrator
+                yield page_name, page_locale, page