Gracefully ignore empty code blocks

resolves #368

src/blacken_docs/__init__.py

@@ -159,6 +159,8 @@ def _rst_match(match: Match[str]) -> str:
         lang = match["lang"]
         if lang is not None and lang not in PYGMENTS_PY_LANGS:
             return match[0]
+        if not match["code"].strip():
+            return match[0]
         min_indent = min(INDENT_RE.findall(match["code"]))
         trailing_ws_match = TRAILING_NL_RE.search(match["code"])
         assert trailing_ws_match
@@ -240,6 +242,8 @@ def _rst_pycon_match(match: Match[str]) -> str:
         if _within_off_range(match.span()):
             return match[0]
         code = _pycon_match(match)
+        if not code.strip():
+            return match[0]
         min_indent = min(INDENT_RE.findall(match["code"]))
         code = textwrap.indent(code, min_indent)
         return f'{match["before"]}{code}'

tests/test_blacken_docs.py

@@ -1323,3 +1323,15 @@ def test_format_src_rst_pycon_comments():
     )
     after, _ = blacken_docs.format_str(before, BLACK_MODE)
     assert after == before
+
+
+def test_format_empty_python_code_block():
+    before = "some text\n\n.. code-block:: python\n\n\nsome other text\n"
+    after, _ = blacken_docs.format_str(before, BLACK_MODE)
+    assert after == before
+
+
+def test_format_empty_pycon_code_block():
+    before = "some text\n\n.. code-block:: pycon\n\n\nsome other text\n"
+    after, _ = blacken_docs.format_str(before, BLACK_MODE)
+    assert after == before