[orca] Return an empty list rather than None if _getXMLRoles() fails

[Joanmarie Diggs <joanied src gnome org>]

commit 7684b839faeca65987e10092c7e51fe2ea329a51
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Mon Oct 15 15:20:14 2018 +0200

    Return an empty list rather than None if _getXMLRoles() fails

 src/orca/scripts/web/script_utilities.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/orca/scripts/web/script_utilities.py b/src/orca/scripts/web/script_utilities.py
index 4266f5a91..48768009f 100644
--- a/src/orca/scripts/web/script_utilities.py
+++ b/src/orca/scripts/web/script_utilities.py
@@ -542,13 +542,13 @@ class Utilities(script_utilities.Utilities):
 
     def _getXMLRoles(self, obj):
         rv = self._xmlRoles.get(hash(obj))
-        if rv is not None:
+        if rv:
             return rv
 
         try:
             attrs = dict([attr.split(':', 1) for attr in obj.getAttributes()])
         except:
-            return None
+            return []
 
         rv = attrs.get('xml-roles', '').split()
         self._xmlRoles[hash(obj)] = rv