Add test for empty "per"

tests/test_main.py

@@ -91,6 +91,13 @@ def test_fraction():
         assert len(tree.warnings) == 1
         assert tree.warnings == [r'The dsi string contains more than one \per, does not match specs! Given string: \metre\per\metre\per\metre']
 
+    # empty fraction
+    with pytest.warns(RuntimeWarning, match=r'The dsi string contains a \\per missing a numerator or denominator! Given string: \\per\\one'):
+        tree = p.parse(r'\per\one')
+        assert tree.toLatex() == r'$$1$$'
+        assert not tree.valid
+        assert len(tree.warnings) == 1
+
 def test_empty():
     with pytest.warns(RuntimeWarning, match='Given D-SI string is empty!'):
         assert p.parse('').toLatex() == ''