Fix null pointer exception in ElementBlogPosts Categories() call (#49)

* Fix null pointer exception in ElementBlogPosts

Add null check for blog object before calling Categories() method.
This prevents "Call to a member function Categories() on null" error
when Blog::get()->byID($val) returns null for non-existent blog IDs.

Fixes issue where attempting to edit an ElementBlogPosts element
with an invalid or deleted blog ID would cause a fatal error.

* Add explicit return statement for null blog case

Address Copilot review feedback by adding explicit return []
when blog is null for better code clarity and consistency.

This makes the control flow more explicit and easier to understand.

Author: jsirish

src/Elements/ElementBlogPosts.php

@@ -76,7 +76,11 @@ public function getCMSFields()
 
                 $dataSource = function ($val) {
                     if ($val) {
-                        return Blog::get()->byID($val)->Categories()->map('ID', 'Title');
+                        $blog = Blog::get()->byID($val);
+                        if ($blog) {
+                            return $blog->Categories()->map('ID', 'Title');
+                        }
+                        return [];
                     }
                     return [];
                 };