fix(Scripts/ICC): Fix crash in npc_darnavan when killer is nil.

[walkline]

Crashlog:

Thread 11 "worldserver" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffda535640 (LWP 338487)]
0x00005555579c2c9d in Object::GetGuidValue (this=0x0, index=<optimized out>) at /opt/wotlk_prod/src/server/game/Entities/Object/Object.cpp:339
339    ASSERT(index + 1 < m_valuesCount || PrintIndexError(index, false));
#0  0x00005555579c2c9d in Object::GetGuidValue (this=0x0, index=<optimized out>) at /opt/wotlk_prod/src/server/game/Entities/Object/Object.cpp:339
#1  0x0000555557cd2963 in Unit::GetCharmerGUID (this=0x0) at /opt/wotlk_prod/src/server/game/Entities/Unit/Unit.h:690
#2  Unit::GetCharmerOrOwnerGUID (this=0x0) at /opt/wotlk_prod/src/server/game/Entities/Unit/Unit.h:1229
#3  Unit::GetCharmerOrOwnerPlayerOrPlayerItself (this=0x0) at /opt/wotlk_prod/src/server/game/Entities/Unit/Unit.cpp:10669
#4  0x000055555684d498 in npc_darnavan::npc_darnavanAI::JustDied (this=0x614000e98c40, killer=0x0) at /opt/wotlk_prod/src/server/scripts/Northrend/IcecrownCitadel/boss_lady_deathwhisper.cpp:1024
#5  0x0000555557cd4e2e in Unit::Kill (killer=<optimized out>, victim=0x626052eb4100, durabilityLoss=<optimized out>, attackType=<optimized out>, spellProto=<optimized out>, spell=<optimized out>) at /opt/wotlk_prod/src/server/game/Entities/Unit/Unit.cpp:18007
#6  0x0000555557cc931d in Unit::DealDamage (attacker=<optimized out>, victim=<optimized out>, damage=<optimized out>, cleanDamage=0x7fffc6e73c90, damagetype=<optimized out>, damageSchoolMask=<optimized out>, spellProto=<optimized out>, durabilityLoss=<optimized out>, damageSpell=<optimized out>) at /opt/wotlk_prod/src/server/game/Entities/Unit/Unit.cpp:1052
#7  0x0000555558ad3655 in AuraEffect::HandlePeriodicDamageAurasTick (this=<optimized out>, target=0xffffffffffffffff, caster=<optimized out>) at /opt/wotlk_prod/src/server/game/Spells/Auras/SpellAuraEffects.cpp:6743

Changes Proposed:

This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

Issues Addressed:

• Closes

SOURCE:

The changes have been validated through:

• Live research (checked on live servers, e.g Classic WotLK, Retail, etc.)
• Sniffs (remember to share them with the open source community!)
• Video evidence, knowledge databases or other public sources (e.g forums, Wowhead, etc.)
• The changes promoted by this pull request come partially or entirely from another project (cherry-pick). Cherry-picks must be committed using the proper --author tag in order to be accepted, thus crediting the original authors, unless otherwise unable to be found

Tests Performed:

This PR has been:

• Tested in-game by the author.
• Tested in-game by other community members/someone else other than the author/has been live on production servers.
• This pull request requires further testing and may have edge cases to be tested.

How to Test the Changes:

• This pull request can be tested by following the reproduction steps provided in the linked issue
• This pull request requires further testing. Provide steps to test your changes. If it requires any specific setup e.g multiple players please specify it as well.

Known Issues and TODO List:

• [ ]
• [ ]

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