Polyhedron_base.interior: Handle the empty polyhedron correctly

Author: Matthias Koeppe

src/sage/geometry/polyhedron/base.py

@@ -8425,7 +8425,15 @@ def interior(self):
             sage: P_lower.interior()
             The empty polyhedron in ZZ^2
 
+        TESTS::
+
+            sage: Empty = Polyhedron(ambient_dim=2); Empty
+            The empty polyhedron in ZZ^2
+            sage: Empty.interior() is Empty
+            True
         """
+        if self.is_open():
+            return self
         if not self.is_full_dimensional():
             return self.parent().element_class(self.parent(), None, None)
         return self.relative_interior()