fix(l4d2-web): show empty-state when overlay dir is empty, not just missing

Tickrate and other seeded examples whose overlay directory exists but
hasn't been built yet rendered a visually blank Files panel — entries
was [] (not None), so the template fell through to an empty <ul>. Use
'not file_tree_root_entries' so both None (dir missing) and []
(dir empty) trigger the 'No files yet' message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/templates/overlay_detail.html

@@ -131,7 +131,7 @@
 
 <section class="panel">
   <h2>Files</h2>
-  {% if file_tree_root_entries is none %}
+  {% if not file_tree_root_entries %}
     <p class="muted">No files yet — build this overlay to populate it.</p>
   {% else %}
     {% set entries = file_tree_root_entries %}

l4d2web/tests/test_overlay_files_routes.py

@@ -324,6 +324,24 @@ def test_overlay_detail_shows_empty_state_when_overlay_dir_missing(
     assert "No files yet" in text
 
 
+def test_overlay_detail_shows_empty_state_when_overlay_dir_is_empty(
+    app, left4me_root: Path
+) -> None:
+    """A built overlay whose directory has been wiped (or seeded but never
+    built) should also fall back to the empty-state message — not render an
+    invisible empty <ul>."""
+    user_id = _make_user()
+    overlay_id = _make_overlay(left4me_root, user_id=user_id, name="my")
+    # _make_overlay leaves the directory in place but empty.
+
+    client = _client_for(app, user_id)
+    response = client.get(f"/overlays/{overlay_id}")
+    text = response.get_data(as_text=True)
+
+    assert response.status_code == 200
+    assert "No files yet" in text
+
+
 def test_overlay_detail_files_section_present_for_workshop_overlays(
     app, left4me_root: Path
 ) -> None: