Empty state message flashes briefly during schema sync in schema view

## Description

When a schema sync is in progress on the schema view, the `NoNodeTypesEmptyState` component briefly flashes (e.g., "No Node Labels Available") before the schema finishes loading and the actual content appears. This creates a jarring visual experience.

The issue was introduced when the `SchemaDiscoveryBoundary` was reordered to show the loading state before checking for existing data. During the transition between the loading spinner completing and the schema data being available, the empty state renders for a frame or two.

## Steps to Reproduce

1. Open the schema view with a connected database
2. Trigger a schema sync (or refresh the page)
3. Observe a brief flash of "No Node Labels Available" before the schema graph renders

## Expected Behavior

The schema view should transition directly from the loading state to the populated schema view without showing the empty state message.

## Related Issues

- Part of #1556
- Result of fixing #1547

Start by reproducing a schema sync in the schema view, then inspect the SchemaDiscoveryBoundary and NoNodeTypesEmptyState transition described in the issue. Trace the loading, existing-data, and empty-state conditions; done means the view moves directly from loading to populated schema content without flashing the empty-state message.

Written by the indexing model from the issue text.