fix(mermaid): render nested transitions with empty labels

[KushalLukhi]

Summary

Fix Mermaid nested graph generation to include transitions even when the trigger/label is an empty string.

This addresses #706 where outer-state transitions were omitted from Mermaid output when labels were empty.

Root cause

NestedGraph._add_edges skipped edges with falsy labels:

if not attr["label"]:
    continue

For transitions like ['', 'A', 'B'], this dropped otherwise valid edges entirely.

Fix

• Keep labeled output as-is: source --> dest: label
• Emit unlabeled edges as source --> dest when label is empty

Tests

Added test_transitions_without_labels_between_outer_states in tests/test_mermaid.py using a minimal nested machine setup from the bug report.

The test asserts that Mermaid output contains:

• C --> A
• A --> B
• B --> C

Validation

• New regression test passes locally.
• Existing targeted Mermaid nested ROI tests also pass.