ament_copyright gives "<unknown>" error

Hello,

When I run the ament_copyright in a colcon test or even manualy inside a terminal in my package with " ament_copyright" I get a warning with no relevant text explanation:

```
2/9 Testing: copyright
2/9 Test: copyright
Command: "/usr/bin/python3" "-u" "/opt/ros/humble/share/ament_cmake_test/cmake/run_test.py" "/colcon_ws/build/my_package_name/test_results/my_package_name/copyright.xunit.xml" "--package-name" "my_package_name" "--output-file" "/colcon_ws/build/my_package_name/ament_copyright/copyright.txt" "--command" "/opt/ros/humble/bin/ament_copyright" "--xunit-file" "/colcon_ws/build/my_package_name/test_results/my_package_name/copyright.xunit.xml"
Directory: /colcon_ws/src/my_package_name
"copyright" start time: Mar 13 09:26 UTC
Output:
----------------------------------------------------------
-- run_test.py: invoking following command in '/colcon_ws/src/my_package_name':
- /opt/ros/humble/bin/ament_copyright --xunit-file /colcon_ws/build/my_package_name/test_results/my_package_name/copyright.xunit.xml
CONTRIBUTING.md: unknown
LICENSE: unknown
2 errors, checked 6 files
```

NOTE: "unknown" above have less_than and greater_than characters before and after, but that caused it to disappear here at save.

I checked also the documentation for ament_copyright , but I found no relevant info about this.

LICENSE file and CONTRIBUTING.md file have:

* UTF-8 encoding
* End in LF
* do not have any special characters.

Thank you!