fix(markdown): skip empty bold headings

tests/test_page_index_md.py

@@ -0,0 +1,23 @@
+import unittest
+
+from pageindex.page_index_md import extract_nodes_from_markdown
+
+
+class ExtractNodesFromMarkdownTest(unittest.TestCase):
+    def test_skips_bold_heading_with_only_whitespace(self):
+        nodes, _ = extract_nodes_from_markdown("**   **\n**Valid heading**")
+
+        self.assertEqual(
+            nodes,
+            [
+                {
+                    "node_title": "Valid heading",
+                    "line_num": 2,
+                    "level": 1,
+                }
+            ],
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()