Ignore Show answer when no song is playing

Before the first song played, currently_playing_song is -1, so pressing
Show answer marked the last song in the list as answered. That is the
preloaded song, so its title was shown as soon as it started playing,
and played?song=-1 was sent to the server. With an empty playlist it
crashed.

tests/unit/test_playlist.gd

@@ -182,3 +182,20 @@ func test_current_titles_are_hidden_until_answered() -> void:
 
     assert_str(Playlist.get_current_soundtrack_title()).is_equal("Soundtrack 0")
     assert_str(Playlist.get_current_song_title()).is_equal("Song 0")
+
+
+func test_answering_when_nothing_is_playing_does_nothing() -> void:
+    Settings.add_to_stats = true
+    var songs: Array[SongObject] = _add_songs(2)
+
+    Playlist.song_is_answered()
+
+    assert_bool(songs[0].is_answered).is_false()
+    assert_bool(songs[1].is_answered).is_false()
+    assert_array(_server.requested_paths).is_empty()
+
+
+func test_answering_with_empty_playlist_does_nothing() -> void:
+    Playlist.song_is_answered()
+
+    assert_array(_server.requested_paths).is_empty()