build: check if fragment_filenames is empty

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

src/towncrier/build.py

@@ -279,6 +279,9 @@ def should_remove_fragment_files(
     answer_yes: bool,
     answer_keep: bool,
 ) -> bool:
+    if not fragment_filenames:
+        click.echo("No news fragments to remove. Skipping...")
+        return False
     try:
         if answer_keep:
             click.echo("Keeping the following files:")