fix(general): handle single and trailing-comma output file paths

[KaoKsn]

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

Fixes Issue: #7645

Description

Fix crashes caused by an empty file path in output_paths when a trailing comma is provided.

    checkov -f target.tf -o json -o junitxml --output-file-path result.json,

Ensure that a single --output-file-path value is treated as a file rather than a directory, so:

    checkov -f target.tf -o json --output-file-path result.json

creates result.json instead of result.json/results_json.json

Add tests covering single output paths and preventing nested output files.

Motivation: Usage lead to finding the bug.

Fixes #7645
Fixes all the problems that issue(#7645) states, expect the output is printed to the console since nothing was suggested for the second problem the issue states.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes