Update example to handle missing content gracefully

Author: jacksleight

docs/examples.md

@@ -43,7 +43,7 @@ use JackSleight\StatamicBardMutator\Facades\Mutator;
 
 Mutator::html('heading', function ($value, $item) {
     if ($item->attrs->level === 2) {
-        $value[1]['id'] = str_slug(collect($item->content)->implode('text', ''));
+        $value[1]['id'] = str_slug(collect($item->content ?? [])->implode('text', ''));
     }
     return $value;
 });
@@ -202,4 +202,4 @@ Statamic includes a set of [modifiers](https://statamic.dev/modifiers) that can
 {{ headings }}
     <a href="#{{ content | bard_text | slugify }}">{{ content | bard_html }}</a>
 {{ /headings }}
-```
+```