This commit adds a comprehensive unit test to validate the fix for PR #6404
that prevents "Strings must not be null" errors when an invalid workflow
entry name is specified.

The test verifies that:
- Using an invalid entry name (e.g., `nextflow run script.nf -entry invalidEntry`)
- Correctly throws IllegalArgumentException with proper error message
- Shows "Unknown workflow entry name: invalidEntry" instead of "Strings must not be null"
- Includes suggestions for valid workflow names when available

The test also includes a fix to ensure null workflow names are filtered
out of the imports section in getWorkflowNames(), making the fix comprehensive
for both local definitions and imports.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>