fix: Add zero state to closed Goal discussions

[Rockyy174]

Fixes #3515.

[sourcery-ai]

Hey - I've left some high level feedback:

• The zero-state rendering logic now duplicates the props.state === "closed" and props.state !== "closed" checks in multiple places; consider centralizing this into a small helper or a single conditional block to make the behavior easier to reason about and maintain.
• The new DiscussionsZeroStateClosed component is very similar to DiscussionsZeroState; you might consider a single configurable zero-state component that takes the message/description as props to avoid duplication.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The zero-state rendering logic now duplicates the `props.state === "closed"` and `props.state !== "closed"` checks in multiple places; consider centralizing this into a small helper or a single conditional block to make the behavior easier to reason about and maintain.
- The new `DiscussionsZeroStateClosed` component is very similar to `DiscussionsZeroState`; you might consider a single configurable zero-state component that takes the message/description as props to avoid duplication.

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