Patch PB bonus projectile knockback (#6784)

# About the pull request

Prevents an issue with PB bonus projectiles.

In a PB, the initial projectile is never "fired", it remains in
nullspace. That's fine, since there's special handling in knockback code
for a projectile in nullspace, it uses the firer.

In a PB, bonus projectiles are set to the target's loc. That doesn't
matter much except for knockback, which uses the position of the
projectile vs. the position of the target. The bonus projectile is on
the same tile as the target, so it can't give a sane angle, so it
defaults to EAST.

This changes the bonus projectile's position to nullspace, matching how
the initial projectile works.

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# Explain why it's good for the game

If a bonus projectile with knockback were ever to be used, this will
prevent issues.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
No player facing changes.

code/modules/projectiles/gun.dm

@@ -1420,7 +1420,7 @@ and you're good to go.
 		if(projectile_to_fire.ammo.bonus_projectiles_amount)
 			var/obj/projectile/BP
 			for(var/i in 1 to projectile_to_fire.ammo.bonus_projectiles_amount)
-				BP = new /obj/projectile(attacked_mob.loc, create_cause_data(initial(name), user))
+				BP = new /obj/projectile(null, create_cause_data(initial(name), user))
 				BP.generate_bullet(GLOB.ammo_list[projectile_to_fire.ammo.bonus_projectiles_type], 0, NO_FLAGS)
 				BP.accuracy = floor(BP.accuracy * projectile_to_fire.accuracy/initial(projectile_to_fire.accuracy)) //Modifies accuracy of pellets per fire_bonus_projectiles.
 				BP.damage *= damage_buff