build: check if fragment_filenames is empty

Co-authored-by: Chris Beaven <smileychris@gmail.com>

src/towncrier/build.py

@@ -263,7 +263,7 @@ def __main(
         click.echo("Staging newsfile...", err=to_err)
         _git.stage_newsfile(base_directory, news_file)
 
-        if should_remove_fragment_files(
+        if fragment_filenames and should_remove_fragment_files(
             fragment_filenames,
             answer_yes,
             answer_keep,