I'm not quite sure what happens with weapons that ran out of ammo and don't have a text on it what to do then (e.g. shotgun). As example: the pistol is clear --> it is stated on the Card that it gets discarded.
The shotgun has no further text what happens after The ammo was used. Does The card remain as ready in play as long as no other weapon is prepared to hold in you hand?
Yes it does. You can always hope to find the extra Ammunition card and reload it. Or you can remove it from the game to make space(remember you only get to have to have two prepared weapons and just because it is empty doesn't mean it's not still prepared...)
Yes the Shotgun stays in play as would Minigun in tve same situation. They can be used again if you find Magazine to reload with. Unlike Minigun tjough the Shotgun can also be destroyed to kill 1 zombie.