Fix list index error when goal_cell and reset_cell are passed as options in PointMaze env

[gauthamvasan]

Description

This PR fixes an assertion error that arises when a specific goal_cell or reset_cell are passed as options in the point_maze env.

Fixes #163

Type of change

Please delete options that are not relevant.

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

Screenshots

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

[rodrigodelazcano]

Thanks a lot @gauthamvasan . Would it be possible to also add a couple of tests for reset_cell and goal_cell? You can add them in the following test files: https://github.com/Farama-Foundation/Gymnasium-Robotics/tree/main/tests/envs/maze

[rodrigodelazcano]

LGTM! Thank you @gauthamvasan