Fix nil error

[Astralcircle]

Fixes:

[[ACF] Armored Combat Framework] lua/acf/damage/damage_sv.lua:37: attempt to perform arithmetic on local 'NewHealth' (a nil value)
1. Network - lua/acf/damage/damage_sv.lua:37
 2. func - lua/acf/damage/damage_sv.lua:228
  3. internal_call - lua/includes/modules/hook.lua:409
   4. Run - lua/includes/modules/hook.lua:428
    5. func - lua/acf/core/globals.lua:230
     6. unknown - lua/includes/extensions/net.lua:38