Fix uninitialized my_array variable in the script Update dco_check.sh

[mdqst]

PR description

This pull request addresses an issue where the my_array variable was not initialized at the beginning of the script. The variable was used for accumulating branch names, but it wasn't explicitly initialized, which could lead to potential errors or unexpected behavior when the script is executed.

Fix Summary:

• Initialization of my_array: The array my_array has now been initialized before its usage. This ensures that the array is properly set up and avoids any issues when appending values to it.
• Script functionality preserved: The script continues to function as expected, checking all branches for commits missing the Signed-off-by line.

Importance of the fix:

• Prevents runtime errors: Without initializing the array, the script could behave unpredictably or throw an error when trying to append values to an uninitialized variable.
• Ensures stability: Proper initialization of variables helps ensure that the script runs consistently across different environments without causing unexpected crashes or issues.

This change improves the reliability of the script and ensures that it will work as intended across various systems.

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests