Sequence puzzle hint signs should be optional

[wjt]

sequence_puzzle_step.gd contains the following:

## An optional sign, showing a hint for this step and whether it has been solved.
@export var hint_sign: SequencePuzzleHintSign:

However it is not optional. If unset, the step node issues this warning:

	if not hint_sign:
		warnings.append("Hint Sign not set")

sequence_puzzle.gd assumes that the hint_sign property of each step is not null, and will crash if any is.

Not all instances of the scene may want to use a hint sign. For instance res://scenes/quests/story_quests/after_the_tremor/2_sequence_puzzle/2_tallerjuego/minijuego2.tscn has purely environmental clues as to the correct sequence to play. There is only one sequence; when you solve it a door opens and a collectible appears. To work around this limitation of the sequence puzzle API, they have a dummy sign way off to the top-right of the scene, where it is not accessible to the player.

It would be nice if the signs were optional. To do this, scenes/game_logic/sequence_puzzle.gd will need to be updated to not assume that each step has a non-null hint_sign property. It might mean moving some state to the sequence_puzzle_step.gd or sequence_puzzle.gd, e.g. tracking which steps are solved.

XP Summary (total 300)

Subdomain	Skill	XP
Engineering: Gameplay & Systems Programming	Problem Solving	75
Engineering: Gameplay & Systems Programming	Creativity & Design Thinking	75
Engineering: Gameplay & Systems Programming	Technical Literacy	150