fix(logic): Fix potential null pointer deferences in GameLogicDispatch

[stephanmeesters]

Scanned through GameLogicDispatch and found some possible null pointer deference issues.

[greptile-apps]

Greptile Summary

This PR fixes potential null pointer dereferences in GameLogicDispatch by adding defensive null checks and making the AIGroup::isEmpty() method const-qualified.

Key changes:

• Made isEmpty() const in both declaration and implementation, enabling it to be called on const AIGroup* pointers
• Added empty group check in getSingleObjectFromSelection() to prevent accessing an empty selection and triggering an incorrect assertion
• Added null check for targetObject in the combat drop message handler before dereferencing it, preventing crashes when the target object doesn't exist

All changes are mirrored correctly between Generals and GeneralsMD (Zero Hour) directories and follow the existing code patterns for null safety in the file.

Confidence Score: 5/5

• This PR is safe to merge with no risk
• All changes are defensive null checks that prevent potential crashes without altering intended program logic. The const qualifier addition is a best practice that enables calling isEmpty() on const pointers. Changes are consistent with existing null-check patterns in the codebase and are correctly mirrored across both game versions.
• No files require special attention

Important Files Changed

Filename	Overview
Generals/Code/GameEngine/Source/GameLogic/System/GameLogicDispatch.cpp	Fixed two null pointer dereference issues: added empty check in getSingleObjectFromSelection() and null check for targetObject before dereferencing in combat drop handler
GeneralsMD/Code/GameEngine/Source/GameLogic/System/GameLogicDispatch.cpp	Fixed two null pointer dereference issues (identical to Generals version)

_{Last reviewed commit: 0be8c37}