feat(#125): skip None artifact in maven step

[h1alexbel]

closes #125

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PR-Codex overview

This PR introduces a new CSV file for testing, modifies the logic in the maven.py file to handle cases where an artifact may not be present, and adds a new test case in test_maven.py to verify the behavior when a plugin lacks an artifact.

Detailed summary

• Added to-maven-without-artifact.csv file with repository and branch information.
• Updated logic in maven.py to check for artifact only if group is None.
• Introduced test_skips_plugin_without_artifact in test_maven.py to test behavior when no artifact is provided.

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