Remove whitespaces of whitespace-only files

Currently, empty and whitespace-only (with or without newlines) are
not modified. In some discussions (issues and pull requests) consesus
was to reformat whitespace-only files to empty or single-character
files, preserving line endings when possible. With that said, this
commit introduces the following behaviors:

* Empty files are left as is
* Whitespace-only files (no newline) reformat into empty files
* Whitespace-only files (1 or more newlines) reformat into a single
newline character

To implement these changes, we moved the initial check at
`format_file_contents` that raises `NothingChanged` if the source
(with no whitespaces) is an empty string. In the case of *.ipynb
files, `format_ipynb_string` checks a similar condition and removed
whitespaces. In the case of Python files, `format_str_once` includes a
check on the output that returns the correct newline character if
possible or an empty string otherwise.

Signed-off-by: Antonio Ossa Guerra <[email protected]>

src/black/__init__.py

@@ -914,9 +914,6 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """
-    if not src_contents.strip():
-        raise NothingChanged
-
     if mode.is_ipynb:
         dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
     else:
@@ -1011,6 +1008,9 @@ def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileCon
     Operate cell-by-cell, only on code cells, only for Python notebooks.
     If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
     """
+    if not src_contents:
+        raise NothingChanged
+
     trailing_newline = src_contents[-1] == "\n"
     modified = False
     nb = json.loads(src_contents)
@@ -1100,6 +1100,11 @@ def _format_str_once(src_contents: str, *, mode: Mode) -> str:
             current_line, mode=mode, features=split_line_features
         ):
             dst_contents.append(str(line))
+    if not dst_contents:
+        normalized_content, _, newline = decode_bytes(src_contents.encode("utf-8"))
+        if "\n" in normalized_content:
+            return newline
+        return ""
     return "".join(dst_contents)