fix: invert NoRtiTrigger condition so mark rti strategy works (#2256)

## Summary
- `NoRtiTrigger::IsActive()` returns `target != nullptr`, meaning it
only fires when a raid icon is **already** assigned — creating a
chicken-and-egg problem where the bot never places the first mark
- Invert to `target == nullptr` so the trigger fires when **no** mark
exists, prompting `MarkRtiAction` to mark the lowest-HP unmarked
attacker
- Once a mark is placed, the trigger stops firing until the marked
target dies

## Test plan
- [ ] Add `mark rti` strategy to a tank bot via `co +mark rti`
- [ ] Enter combat with multiple mobs — bot should auto-mark lowest HP
target with skull
- [ ] Verify mark persists until target dies, then bot marks next target
- [ ] Verify no marking occurs out of combat

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Keleborn <22352763+Celandriel@users.noreply.github.com>
Co-authored-by: bash <hermensb@gmail.com>
Co-authored-by: Revision <tkn963@gmail.com>
Co-authored-by: kadeshar <kadeshar@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Ai/Base/Trigger/RtiTriggers.cpp

@@ -16,5 +16,5 @@ bool NoRtiTrigger::IsActive()
         return false;
 
     Unit* target = AI_VALUE(Unit*, "rti target");
-    return target != nullptr;
+    return target == nullptr;
 }