[Fix-18448][ui] Fix releaseState undefined access in dag computed properties

[nanxiuzi]

Summary

Fixes #18448

dag/index.tsx has two computed properties (startDisplay and menuDisplay) that access props.definition.workflowDefinition.releaseState without null-checking workflowDefinition. When workflowDefinition is undefined (e.g., the workflow is in a draft state, the definition was deleted but task instances still reference it, or the API response is missing the field), the access throws TypeError: Cannot read properties of undefined (reading 'releaseState') and breaks the DAG editor page.

Changes

• dolphinscheduler-ui/src/views/projects/workflow/components/dag/index.tsx: replace props.definition!.workflowDefinition.releaseState with props.definition?.workflowDefinition?.releaseState in two computed properties (startDisplay at line 130, menuDisplay at line 152).

This matches the safe pattern already used in dag-toolbar.tsx (lines 276 and 512): props.definition?.workflowDefinition?.releaseState. No behavior change when workflowDefinition is defined — only the previously-crashing case now returns false (the same value the else branches already return).

Verification

• pnpm run build:prod passes (vue-tsc type check + Vite production build).
• Manual test on a 3.4.2 deployment: previously, clicking "Edit" on a task in the affected state produced Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'releaseState') in the browser console and the page stopped rendering. With this fix, the page renders correctly and the right-click menu items (startDisplay, menuDisplay) are hidden when workflowDefinition is undefined, which is the correct behavior since the user cannot meaningfully start/operate on a workflow that has no workflowDefinition.

Reproduce steps (for reviewers)

1. Open a workflow definition in a state where props.definition.workflowDefinition is undefined (e.g., draft, or workflow definition just deleted but task instances still referencing it).
2. Right-click a task node → "Edit".
3. Without this PR: browser console shows Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'releaseState') and the DAG page stops rendering.
4. With this PR: the page renders normally; the start button and "other" menu are hidden in the right-click menu (since workflowDefinition is undefined).