[Bugfix]--Add default entries into configuration

[jturner65]

Motivation and Context

This PR adds 2 missing default (empty string) entries (for "navmesh_instance" and "semantic_scene_instance") in SceneInstanceAttributes so the configuration will not throw an error if these values are not present when queried.

How Has This Been Tested

C++ and python tests pass locally

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.