Update CHANGES.md

Add entry about new behavior on whitespace-only files: removing
whitespace characters and return a single newline (if present) or an
empty file

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

CHANGES.md

@@ -29,6 +29,8 @@
 - Parsing support has been added for walruses inside generator expression that are
   passed as function args (for example,
   `any(match := my_re.match(text) for text in texts)`) (#3327).
+- Reformat empty and whitespace-only files as either an empty file (if no newline is
+  present) or as a single newline character (if a newline is present) (#3348)
 
 ### Performance