Update fn_SUP_CASRoutine.sqf

[Rythoka]

Fix HandleDamage event returning scriptHandlers when CAS plane is struck by bullets

What type of PR is this.

1. Bug
2. Change
3. Enhancement

What have you changed and why?

Added a single nil line to the end of the HandleDamage EH to make sure the EH always returns Nothing, which is necessary for damage to be applied normally per https://community.bistudio.com/wiki/Arma_3:_Event_Handlers#HandleDamage

Please specify which Issue this PR Resolves.

closes #1938

Please verify the following and ensure all checks are completed.

1. Have you loaded the mission in LAN host?
2. Have you loaded the mission on a dedicated server?

Is further testing or are further changes required?

1. No
2. Yes (Please provide further detail below.)

How can the changes be tested?

Steps:

1. Spawn an empty ZSU-39 and get in
2. Make ZSU-39 and units inside invincible (e.g. with Zeus)
3. Spawn a CAS support onto the player's position
4. When the CAS arrives, strike it with the ZSU-39's autocannon
5. Check the CAS plane's health with Zeus

Current behavior: CAS plane is undamaged by autocannon
Fixed behavior: CAS plane takes damage

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Notes:

[HakonRydland]

while the wiki claims only when returning a numeric value would you overwrite damage, i would not be surprised if this is wrong.

one note, pls fix indentation of the nil, it should be one further in.