Fix NullPointerException in nf-tower plugin when onFlowBegin fails

Summary

Add defensive null checking for sender thread and workflowId to prevent NullPointerException when onFlowComplete is called after initialization failures in onFlowCreate or onFlowBegin methods.

• Add null checks for sender thread and workflowId in TowerClient.onFlowComplete()
• Prevents NullPointerException when Tower API rejects configuration with HTTP 400
• Fixes workflow failure cascade caused by early exit from onFlowBegin hook

Problem

When Nextflow encounters invalid configuration (e.g., #6382 ), the following cascade occurs:

1. Nextflow sends malformed configuration to Seqera Platform via onFlowBegin hook
2. Platform rejects configuration and returns HTTP 400 response
3. nf-tower plugin receives 400 response and throws AbortOperationException in onFlowBegin
4. Early exit from onFlowBegin prevents initialization of sender thread and workflowId
5. When workflow completes, onFlowComplete() calls sender.join() on null reference
6. NullPointerException causes workflow to fail, masking the original configuration issue

This was reported in community forum thread and Seqera internal ticket #6701, where users experienced "Workflow execution completed unsuccessfully" despite successful pipeline runs, due to Tower communication failures caused by configuration parsing issues.

Solution

Added defensive null checks in onFlowComplete():

• sender thread check: Skip thread join if sender was never initialized due to onFlowBegin failure
• workflowId check: Skip completion HTTP request if workflow ID was never set due to onFlowCreate failure

These changes ensure that initialization failures don't cause secondary NullPointerExceptions that mask the root cause, allowing users to see the actual configuration errors.

Test Plan

• Verify existing nf-tower plugin tests pass
• Confirm project compilation succeeds
• Test with malformed configuration to ensure graceful handling (not necessary)
• Verify workflow completion reporting works correctly after configuration fixes (not necessary)

Related Issues

• Community forum: https://community.seqera.io/t/nextflow-not-communicating-back-to-seqera/2353
• Ticket Add onWorkflowPublish event to TraceObserverV2 #6701: NullPointerException in Tower communication due to configuration parsing failures