M: Missing validation for input files - #6

# Description

The `challenge-m.sh` script assumes that all required input files both **exist** and **contain data**.

When one or more required files are **missing** or **empty**, the script currently:

* fails with unclear errors, or
* appears to succeed while doing nothing useful.

The script must explicitly validate its inputs before any processing starts.

## Acceptance Criteria

* [x] Check that all required input files exist **before** they are used
* [ ] Check that all required input files are **not empty**
* [ ] If any validation fails:

* [ ] Print a clear, human-readable error message
* [ ] Exit the script with a non-zero exit code
* [ ] No file processing should happen if validation fails

## Notes

* Do **not** assume file existence implies valid data
* Validation should happen at the **top of the script**
* Error messages should explain **what is wrong** and **what file is affected**