fix(detect-secrets): add case for non utf-8 values

[pedrooot]

Context

Partially Fixes #6728

Description

This pull request includes updates to handle cases where environment variables containing secrets are not found, by providing a default value of 'UNKNOWN'. Additionally, new test cases have been added to verify these changes.

Error handling improvements:

• prowler/providers/aws/services/awslambda/awslambda_function_no_secrets_in_variables/awslambda_function_no_secrets_in_variables.py: Updated the secret detection logic to return 'UNKNOWN' if the environment variable is not found.
• prowler/providers/aws/services/ecs/ecs_task_definitions_no_environment_secrets/ecs_task_definitions_no_environment_secrets.py: Modified the secret detection logic to return 'UNKNOWN' if the environment variable is not found.

New test cases:

• tests/providers/aws/services/awslambda/awslambda_function_no_secrets_in_variables/awslambda_function_no_secrets_in_variables_test.py: Added a test case to ensure that 'UNKNOWN' is returned when the environment variable is not found.
• tests/providers/aws/services/ecs/ecs_task_definitions_no_environment_secrets/ecs_task_definitions_no_environment_secrets_test.py: Added a test case to verify that 'UNKNOWN' is returned when the environment variable is not found.

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.
• Review if is needed to change the Readme.md

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.74%. Comparing base (627c115) to head (184cdec).
Report is 41 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6758      +/-   ##
==========================================
- Coverage   88.75%   88.74%   -0.01%     
==========================================
  Files        1195     1195              
  Lines       34488    34488              
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- Hits        30610    30607       -3     
- Misses       3878     3881       +3     

Flag	Coverage Δ
prowler	88.74% <ø> (-0.01%)	⬇️

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Components	Coverage Δ
prowler	88.74% <ø> (-0.01%)	⬇️
api	∅ <ø> (∅)

[pedrooot]

Closed by #6710