Accept empty stdin (close #2337)

src/black/__init__.py

@@ -803,7 +803,8 @@ def format_stdin_to_stdout(
         )
         if write_back == WriteBack.YES:
             # Make sure there's a newline after the content
-            dst += "" if dst[-1] == "\n" else "\n"
+            if dst and dst[-1] != "\n":
+                dst += "\n"
             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
             now = datetime.utcnow()

tests/test_black.py

@@ -1679,6 +1679,19 @@ def test_reformat_one_with_stdin_and_existing_path(self) -> None:
             # __BLACK_STDIN_FILENAME__ should have been stripped
             report.done.assert_called_with(expected, black.Changed.YES)
 
+    def test_reformat_one_with_stdin_empty(self) -> None:
+        """Check that black doesn't fail for empty stdin (issue/#2337)"""
+        with patch("sys.stdout", open(os.devnull, "w")):
+            # Patch `sys.stdout` to null device lest
+            # `format_stdin_to_stdout` detaches stdout at the end and causes
+            # "ValueError: I/O operation on closed file" in subsequent tests
+            black.format_stdin_to_stdout(
+                fast=True,
+                content="",
+                write_back=black.WriteBack.YES,
+                mode=DEFAULT_MODE,
+            )
+
     def test_gitignore_exclude(self) -> None:
         path = THIS_DIR / "data" / "include_exclude_tests"
         include = re.compile(r"\.pyi?$")