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/integration/test_main_window.gd

@@ -214,3 +214,15 @@ func test_closing_settings_with_full_cache_downloads_nothing() -> void:
     _close_settings()
 
     assert_int(_server.count("/music/rand")).is_equal(downloads_before)
+
+
+func test_show_answer_before_first_song_does_not_reveal_next_song() -> void:
+    _press("ShowAnswerButton")
+
+    assert_bool(Playlist.song_object_array[0].is_answered).is_false()
+    assert_int(_server.count("/music/played?song=-1")).is_equal(0)
+
+    await _play_first_song()
+
+    assert_str((_window.get_node("VBoxContainer/SoundtrackLabel") as Label).text) \
+            .is_equal("????????")