build: fix check_autogenerated_edits.py flagging up files that didn't exist

Before this change new backend docs would have their changes flagged
which is undesirable for the first revision.

bin/check_autogenerated_edits.py

@@ -77,6 +77,10 @@ def check_file(file):
     viol = False
     new_lines = get_file_content("HEAD", file)
     old_lines = get_file_content("HEAD~1", file)
+
+    # If old file did not exist or was empty then don't check
+    if not old_lines:
+        return
     
     # Entire autogenerated file check.
     if any("autogenerated - DO NOT EDIT" in l for l in new_lines[:10]):