Raider from Pyke says: Each Weapon card in your hand gains ambush (X). X is that card's printed cost.

Pinch of Powder is a Weapon attachment with -- cost, but it has Shadow(1).

From rules on Shadow cards with -- cost: "Such a card cannot be marshaled or played from a player’s hand, and may only be played by being brought out of shadows. Such a card has no printed cost. If an ability references the printed cost of that card, a null value is returned."

Questions:

1) Is a null value equivalent to 0, when X targets it? So does Pinch of Powder gain Ambush (0)?

2) If so, assuming I have a Raider from Pyke in play, Can I ambush in Pinch of Powder for 0 Gold? Or does 'such a card cannot be ... played from a player's hand, and may only be played by being brought out of Shadows', prevent Pinch of Powder from being ambushed in?

1. When an X is not defined (including by referencing a null value, see The Hand's Judgment), it is 0.
2. Ambush (like Shadow) is way to put cards into play, which is distinct from both "marshal" (referring to characters, locations and attachments being paid for during the marshaling phase) and "play" (referring to events).

So the rule about "-" costs doesn't prevent cards from being ambushed, and the rule about X means Pinch of Powder can be ambushed for 0 thanks to Raider from Pyke.