CorrelationStrategy blocks all workflow instances when CorrelationId is null

## Description

When a workflow uses **Activation Strategy = Correlation**, Elsa blocks new workflow instances from starting even when they should be allowed to run. This happens specifically when the `CorrelationId` is `null`, which occurs when executing workflows from the Elsa Studio UI.

## Environment
- **Elsa Version:** 3.x (current)
- **Execution Context:** Elsa Studio UI

## Root Cause

The issue is in the `CorrelationStrategy` class implementation:

**File:** `src/modules/Elsa.Workflows.Runtime/ActivationValidators/CorrelationStrategy.cs`

```csharp
public async ValueTask GetAllowActivationAsync(WorkflowInstantiationStrategyContext context)
{
var filter = new WorkflowInstanceFilter
{
CorrelationId = context.CorrelationId, // ← This is null when executed from UI
WorkflowStatus = WorkflowStatus.Running
};

var count = await workflowInstanceStore.CountAsync(filter);
return count == 0;
}
```

When `context.CorrelationId` is `null`, the filter matches **ALL running workflow instances** (of any correlation), not just instances with the same correlation ID. This causes Elsa to block new instances if **any** instance of any workflow is currently running.

## Steps to Reproduce

1. Create a workflow in Elsa Studio
2. Set **Activation Strategy** to **"Correlation"**
3. Publish the workflow
4. Run the workflow from Elsa Studio UI (using the Run button)
5. Try to run it a second time while the first instance is still running or some other instance is running

**Expected Behavior:**
Since no `CorrelationId` is provided (or it's unique), a new instance should be allowed to start.

**Actual Behavior:**
The workflow is blocked with error: "The workflow cannot be started"

## Impact

- **Workflows cannot be tested from Elsa Studio UI** when using Correlation strategy
- Any API call without explicit `CorrelationId` parameter will be blocked if any workflow instance is running
- This defeats the purpose of the Correlation strategy, which should only block instances with the **same** correlation ID

Start by reading src/modules/Elsa.Workflows.Runtime/ActivationValidators/CorrelationStrategy.cs, especially GetAllowActivationAsync, and trace how WorkflowInstanceFilter handles a null CorrelationId. Reproduce the Elsa Studio workflow steps from the issue, then verify that a null or unique correlation ID does not block activation because of unrelated running instances.

Written by the indexing model from the issue text.