[orca] Refine substring heuristic for mouse review duplicate check

[Joanmarie Diggs <joanied src gnome org>]

commit 5ba39d4b2292cf39e7e7f6f215de5152b5206ca3
Author: Joanmarie Diggs <jdiggs igalia com>
Date:   Tue Jul 9 11:07:14 2019 -0400

    Refine substring heuristic for mouse review duplicate check

 src/orca/mouse_review.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/orca/mouse_review.py b/src/orca/mouse_review.py
index 60cc9d7ad..0a980e331 100644
--- a/src/orca/mouse_review.py
+++ b/src/orca/mouse_review.py
@@ -178,7 +178,7 @@ class _ItemContext:
             debug.println(debug.LEVEL_INFO, msg, True)
             return False
 
-        if not self._isSubstringOf(prior):
+        if self.getString() and prior.getString() and not self._isSubstringOf(prior):
             msg = "MOUSE REVIEW: Not a duplicate: not a substring of"
             debug.println(debug.LEVEL_INFO, msg, True)
             return False