Fixes single-env handling of terrain out of bounds termination

[KyleM73]

Description

The terrain_out_of_bounds termination condition creates an error with single environments when called on an infinite plane because it returns the boolean False. This PR replaces the return in this case with a boolean tensor of size (num_envs,).

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Mayankm96]

Thanks for the MR! Closing since it is duplicate of: #3180