fix: terrain hlod null reference exception

[AymanDev]

Purpose of this PR

NullReferenceException when no Normal map or Mask map provided in TerrainHLOD

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Release Notes

• Implemented new Create method in TerrainHLODCreator for Action callback when HLOD creation completed.
• Made public options for TerrainHLOD

This helped me automate creating via custom editor gui create HLODs for multiple terrains at once.

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Testing status

Tested this on my project in Unity 2023.3.18f1

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Overall Product Risks

Technical Risk: None

Halo Effect: None

No new features from ground up was created. Only few fixes and callback.

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