IStartupTask Not Run When No Tenants Are Available

### Problem Description

When multitenancy is enabled with a tenants provider that returns **zero tenants** (e.g., `ConfigurationTenantsProvider` with empty `Multitenancy:Tenants`), the following occurs:

- `ActivateTenants` hosted service runs `ActivateTenantsAsync`
- `DefaultTenantService.RefreshAsync()` / `GetTenantsDictionaryAsync()` receives an empty tenant list
- The `foreach (var tenant in tenants)` loop never executes
- `RegisterTenantAsync()` is never called
- `TenantActivatedAsync` is never invoked
- **No startup tasks run**, including `PopulateRegistriesStartupTask`, `RunMigrationsStartupTask`, `UpdateRouteTableStartupTask`, etc.

### Expected Behavior

- When multitenancy is enabled but no tenants are configured, the system should **at least activate `Tenant.Default`** (`Tenant.DefaultTenantId = ""`) so that:
1. Core startup tasks execute (registry population, migrations, route updates)
2. The application can operate in a "single default tenant" mode until tenants are added

### Affected Code Paths

```
ActivateTenants.StartAsync
→ ITenantService.ActivateTenantsAsync
→ DefaultTenantService.RefreshAsync / GetTenantsDictionaryAsync
→ foreach (var tenant in tenants) // Never executes if empty
→ RegisterTenantAsync
→ tenantEvents.TenantActivatedAsync
→ TenantTaskManager.TenantActivatedAsync
→ RunStartupTasksAsync // Never reached
→ IStartupTask.ExecuteAsync (PopulateRegistriesStartupTask, etc.)
```

Start at ActivateTenants.StartAsync and trace ITenantService.ActivateTenantsAsync through DefaultTenantService.RefreshAsync and GetTenantsDictionaryAsync. Verify the empty-tenant path reaches tenant activation and startup-task execution, and confirm that the default tenant and listed startup tasks operate correctly when no tenants are configured.

Written by the indexing model from the issue text.