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

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

tests/test_backend_markdown.py

@@ -0,0 +1,35 @@
+from pathlib import Path
+
+from docling.backend.md_backend import MarkdownDocumentBackend
+from docling.datamodel.base_models import InputFormat
+from docling.datamodel.document import InputDocument
+
+
+def test_convert_valid():
+    fmt = InputFormat.MD
+    cls = MarkdownDocumentBackend
+
+    test_data_path = Path("tests") / "data"
+    relevant_paths = sorted((test_data_path / "md").rglob("*.md"))
+    assert len(relevant_paths) > 0
+
+    for in_path in relevant_paths:
+        gt_path = test_data_path / "groundtruth" / "docling_v2" / f"{in_path.name}.md"
+
+        in_doc = InputDocument(
+            path_or_stream=in_path,
+            format=fmt,
+            backend=cls,
+        )
+        backend = cls(
+            in_doc=in_doc,
+            path_or_stream=in_path,
+        )
+        assert backend.is_valid()
+
+        act_doc = backend.convert()
+        act_data = act_doc.export_to_markdown()
+
+        with open(gt_path, "r", encoding="utf-8") as f:
+            exp_data = f.read().rstrip()
+        assert act_data == exp_data