[pygobject] gtk overrides: Fix return type Gtk.TreePath with depth == 0. Fixes #143

[Christoph Reiter <creiter src gnome org>]

commit e4d7675b0e80826db2c1b890ff11fa6ded8394d2
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Sun Mar 18 07:33:19 2018 +0100

    gtk overrides: Fix return type Gtk.TreePath with depth == 0. Fixes #143
    
    compare() also errors out with depth == 0, but str() shouldn't fail
    as it's important for debugging.

 gi/overrides/Gtk.py         | 2 +-
 tests/test_overrides_gtk.py | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/gi/overrides/Gtk.py b/gi/overrides/Gtk.py
index e15a0bb7..71001832 100644
--- a/gi/overrides/Gtk.py
+++ b/gi/overrides/Gtk.py
@@ -1193,7 +1193,7 @@ class TreePath(Gtk.TreePath):
         super(TreePath, self).__init__()
 
     def __str__(self):
-        return self.to_string()
+        return self.to_string() or ""
 
     def __lt__(self, other):
         return other is not None and self.compare(other) < 0
diff --git a/tests/test_overrides_gtk.py b/tests/test_overrides_gtk.py
index fbfe9cfb..e7d15942 100644
--- a/tests/test_overrides_gtk.py
+++ b/tests/test_overrides_gtk.py
@@ -1401,6 +1401,10 @@ class TestTreeModel(unittest.TestCase):
         self.assertEqual(p1[2], 3)
         self.assertRaises(IndexError, p1.__getitem__, 3)
 
+    def test_tree_path_empty(self):
+        p1 = Gtk.TreePath.new()
+        assert str(p1) == ""
+
     def test_tree_model(self):
         tree_store = Gtk.TreeStore(int, str)