Prevent Exception (joomla#40122)

* Prevent Exception

If $parts is `null` this will throw an exception.

```count(): Argument #1 ($value) must be of type Countable|array, null given```

This can happen if the context is e.g. the default context `text` from the `Joomla\CMS\HTML\Helpers\Content::prepare()` function.

* Update fields.php

plugins/content/fields/src/Extension/Fields.php

@@ -91,7 +91,7 @@ private function prepare($string, $context, $item)
 
         $parts = FieldsHelper::extract($context);
 
-        if (count($parts) < 2) {
+        if (!$parts || count($parts) < 2) {
             return $string;
         }