fix: emit a warning when no content is rendered

Fixes #149

Signed-off-by: Mike Fiedler <[email protected]>

Author: miketheman

readme_renderer/rst.py

@@ -129,4 +129,5 @@ def render(
     if rendered:
         return clean(rendered)
     else:
+        stream.write("no output rendered")
         return None

tests/test_rst.py

@@ -53,3 +53,13 @@ def test_rst_raw():
 """, stream=warnings) is None
 
     assert '"raw" directive disabled' in warnings.getvalue()
+
+
+def test_rst_header_only():
+    warnings = io.StringIO()
+    assert render("""
+Header
+======
+""", stream=warnings) is None
+
+    assert "no output rendered" in warnings.getvalue()