Issue #21: Missing Null Checks

### **Issue #21: Missing Null Checks**  
**Type**: Contract Violation  
**Files**: BlogPost.cs  

#### **Problematic Code**:
```c#
public BlogPost(string title) // No null validation
{
    Title = title; // Potential NRE
}
```

**Solution**:
```c#
public BlogPost(string title)
{
    Title = title ?? throw new ArgumentNullException(nameof(title));
}
```
