g.extension: fix install_extension_xml() when no metadata available

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@54711 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/g.extension/g.extension.py

@@ -605,7 +605,7 @@ def install_extension_xml(url, mlist):
 
     if not data:
         grass.warning(_("No metadata available"))
-        return
+        return []
 
     fXML = os.path.join(options['prefix'], 'modules.xml')
     # create an empty file if not exists