feat: handle blank canvas state in Diagram Editor

[lornakelly]

Description

The diagram editor currently does not handle a blank/empty canvas state. When the text editor is initialised with no content (blank state), the diagram fails to render appropriately or may show errors.

Expected Behaviour

• Diagram should display an appropriate empty state when no workflow content is present
• Empty state should provide helpful guidance (e.g., "Start by adding workflow content in the text editor")
• No errors should be thrown when initialising with blank content
• Smooth transition from empty state to populated diagram when content is added

Design TBD

Motivation

Supporting a blank canvas state is critical for delivering a complete authoring experience, particularly for The Newcomer persona

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Open Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.