fix(ui): show empty state when no model configs exist (#1930)

[gauravshinde1729]

---

Title:
fix(ui): show empty state when no model configs exist (#1930)

Description:

What

When no ModelConfig resources are deployed, the UI showed a
"Failed to fetch models" error instead of rendering the normal empty
state ("no agents yet" / empty models page).

Fixes #1930

Why

The backend's StandardResponse.Data field uses json:"data,omitempty",
so an empty list serializes with the data field omitted entirely
(e.g. {"error":false,"message":"Successfully listed ModelConfigs"}),
returned with HTTP 200.

The UI treated a missing data as a fetch failure:

if (!response.data || response.error) {
  throw new Error(response.error || "Failed to fetch models");
}

So a valid "zero model configs" response was misread as an error and
surfaced via the shared error state in AgentsProvider (and on the
/models page).

Change

Treat a missing/empty list as a valid empty result, and only fail on an
explicit response.error — matching the existing defensive pattern in
AgentList (result.data || []):

• ui/src/components/AgentsProvider.tsx — fetchModels
• ui/src/app/models/page.tsx — fetchModels

This is a frontend-only fix; the backend omitempty behavior is
unchanged.

Screenshots

Before:

After: