Disambiguate `ShockwaveDamage` cases

[Baezon]

If a weapon's shockwave damage has not been specified via table, ShockwaveDamage should return -1 instead of 0, and otherwise return the value set via table, allowing scripts to disambiguate explicitly 0 and unspecified. Additionally, a few parsing changes to ensure shockwave damage can never genuinely be negative (all damage handling is already doing this so this will have no gameplay effects).

This is an explicit, unflagged change to the specification of this script value, but it's really not worth a flag, and a very niche case for which backwards compatibility concerns would surely be negligible.