Federated Catalog is not loading, if there is a empty datasets

# Bug Report

## Describe the Bug

If the federated catalog endpoint of a connector is used and one of the queried catalogs does not have any asset, the datasets attribute in that catalog is missing, which leads to an endless loading screen.

### Expected Behavior

The empty catalog will be skipped, and the overall federated catalog will be displayed.

### Observed Behavior

Endless loading screen.

## Steps to Reproduce

Steps to reproduce the behavior:

Provide a connector with an empty catalog.

Using this connector configuration:
```json
{
"connectorName": "tester",
"managementUrl": "http://localhost:8080/api/management",
"defaultUrl": "http://localhost:8080/api",
"protocolUrl": "http://localhost:8080/api/v1/dsp",
"federatedCatalogEnabled": true,
"federatedCatalogUrl": "http://localhost:8080/api/catalog",
"did": ""
}
```

1. Go to 'Catalog'
2. See error

## Context Information

Used version: EDC v0.14.0

## Possible Implementation

The catalog service should add a check to verify whether the datasets attribute exists before attempting to flatten it.