Extra null safety

Co-Authored-By: Alexandre Flament <alex.andre@al-f.net>

searx/webapp.py

@@ -354,7 +354,8 @@ def custom_url_for(endpoint: str, override_theme: Optional[str] = None, **values
             theme_name = get_current_theme_name(override=override_theme)
             filename_with_theme = "themes/{}/{}".format(theme_name, values['filename'])
             file_hash = static_files.get(filename_with_theme)
-            values['filename'] = filename_with_theme
+            if file_hash:
+                values['filename'] = filename_with_theme
         if get_setting('ui.static_use_hash') and file_hash:
             suffix = "?" + file_hash
     if endpoint == 'info' and 'locale' not in values: