RST writer: add empty comments when needed...

to avoid including a blocquote in the indented content
of a preceding block.

Closes #3675.

test/command/3675.md

@@ -0,0 +1,15 @@
+````
+% pandoc -t rst
+```python
+print("hello")
+```
+> block quote
+^D
+.. code:: python
+
+    print("hello")
+
+..
+
+    block quote
+````