Add additional test for non-existing string path

tests/test_urdf_parsing.py

@@ -1,3 +1,5 @@
+import pathlib
+
 import pytest
 import robot_descriptions
 import robot_descriptions.loaders.idyntree
@@ -37,9 +39,12 @@ def test_urdf_parsing(robot: Robot) -> None:
 
     # Check that it fails is is_urdf=True and the resource is a non-existing path
     with pytest.raises(FileNotFoundError):
-        _ = rod.Sdf.load(sdf="/non/existing/path", is_urdf=None)
         _ = rod.Sdf.load(sdf="/non/existing/path", is_urdf=False)
 
+    # Check that it fails is is_urdf=True and the resource is an empty string
+    with pytest.raises(FileNotFoundError):
+        _ = rod.Sdf.load(sdf=pathlib.Path("/non/existing/path"), is_urdf=False)
+
     # The following instead should succeed
     _ = rod.Sdf.load(sdf=urdf_path, is_urdf=None)
     _ = rod.Sdf.load(sdf=urdf_path, is_urdf=True)