fix: fix single newline handling in MD backend (#824)

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

tests/data/groundtruth/docling_v2/duck.md.md

@@ -0,0 +1,52 @@
+Summer activities
+
+# Swimming in the lake
+
+Duck
+
+Figure 1: This is a cute duckling
+
+## Let’s swim!
+
+To get started with swimming, first lay down in a water and try not to drown:
+
+- You can relax and look around
+- Paddle about
+- Enjoy summer warmth
+
+Also, don’t forget:
+
+- Wear sunglasses
+- Don’t forget to drink water
+- Use sun cream
+
+Hmm, what else…
+
+## Let’s eat
+
+After we had a good day of swimming in the lake, it’s important to eat something nice
+
+I like to eat leaves
+
+Here are some interesting things a respectful duck could eat:
+
+|         | Food                             |   Calories per portion |
+|---------|----------------------------------|------------------------|
+| Leaves  | Ash, Elm, Maple                  |                     50 |
+| Berries | Blueberry, Strawberry, Cranberry |                    150 |
+| Grain   | Corn, Buckwheat, Barley          |                    200 |
+
+And let’s add another list in the end:
+
+- Leaves
+- Berries
+- Grain
+
+And here my listing in code:
+
+```
+Leaves
+
+Berries
+Grain
+```