Fix possibly unbound Body by assigning preassigning them the else case and overriding them in the if case.

The current case will work until eiether the has_chunk_header above or below the _process_diff function changes, and then it will cause runtime crash as one cannot concat string and None.

Change-Id: I81737328f609e636bcee5087a2bc4aa96a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7260374
Reviewed-by: Brian Egizi <begizi@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Alex Ovsienko <ovsienko@google.com>

presubmit_diff.py

@@ -84,11 +84,10 @@ def _process_diff(diff: str, src_root: str, dst_root: str) -> str:
         return ""
 
     has_chunk_header = HEADER_DELIMITER in diff
+
+    header, body = diff, ""
     if has_chunk_header:
         header, body = diff.split(HEADER_DELIMITER, maxsplit=1)
-    else:
-        # Only the file mode changed.
-        header = diff
 
     norm_src = src_root.rstrip(os.sep)
     norm_dst = dst_root.rstrip(os.sep)