[orca] Return an empty string rather than None when retrieving a doc's URI

[Joanmarie Diggs <joanied src gnome org>]

commit 8ff41be58c8c65b69d175c54e37502c79411f2c3
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Tue Apr 9 10:43:46 2019 -0400

    Return an empty string rather than None when retrieving a doc's URI

 src/orca/scripts/web/script_utilities.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/orca/scripts/web/script_utilities.py b/src/orca/scripts/web/script_utilities.py
index 6be3be541..ab47907a7 100644
--- a/src/orca/scripts/web/script_utilities.py
+++ b/src/orca/scripts/web/script_utilities.py
@@ -312,7 +312,7 @@ class Utilities(script_utilities.Utilities):
             else:
                 return document.getAttributeValue('DocURL') or document.getAttributeValue('URI')
 
-        return None
+        return ""
 
     def isPlainText(self, documentFrame=None):
         return self.mimeType(documentFrame) == "text/plain"