Default pointmaze maps to empty

[rodrigodelazcano]

Description

Default the maps of the simple point maze environments to empty (PointMaze_Open-v3, PointMaze_UMaze-v3, PointMaze_Medium-v3, PointMaze_Large-v3). This way the initial location of the agent and of the goal will be randomly selected among the empty cells (value 0 in the map representation) when the environment is reset.

Type of change

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• 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 not work as expected)
• This change requires a documentation update

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Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• 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
• New and existing unit tests pass locally with my changes