fix: prevent nil pointer panic when mission init fails

[fank]

Summary

• Add nil check in EndMission() to return error instead of panic when no mission was started
• Add nil check in GetExportMetadata() to return empty metadata instead of panic
• Add tests for both nil mission scenarios

Problem

When :INIT: fails (e.g., Arma addon passes any instead of an array), no mission is started. However, subsequent commands like :SAVE:MISSION: still execute. When EndMission() or GetExportMetadata() tried to access b.mission fields, it caused a nil pointer dereference panic:

["error", "panic in RVExtensionArgs: runtime error: invalid memory address or nil pointer dereference"]

Test plan

• TestEndMissionWithoutStartMission - verifies error returned instead of panic
• TestGetExportMetadataWithoutStartMission - verifies empty metadata returned instead of panic
• All existing tests pass

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/OCAP2/extension/v5/internal/storage/memory	14.85% (+0.07%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/OCAP2/extension/v5/internal/storage/memory/memory.go	16.81% (+0.09%)	1410 (+40)	237 (+8)	1173 (+32)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/OCAP2/extension/v5/internal/storage/memory/memory_test.go