Add section about using if guards in documentTemplate guide (#3347)

Author: CaroFG

learn/ai_powered_search/document_template_best_practices.mdx

@@ -56,10 +56,25 @@ In the sample document, the `overview` alone is 49 words. Use Liquid's [`truncat
 
 Short prompts do not have enough information for the embedder to properly understand the query context. Long prompts instead provide too much information and make it hard for the embedder to identify what is truly relevant about a document.
 
+## Add guards for missing fields
+
+Some documents might not contain all the fields you expect. If your template directly references a missing field, Meilisearch will throw an error when indexing documents.
+
+To prevent this, use Liquid’s `if` statements to add guards around fields:
+
+```
+{% if doc.title %}
+A movie called {{ doc.title }}
+{% endif %}
+```
+
+This ensures the template only tries to include data that already exists in a document. If a field is missing, the embedder still receives a valid and useful prompt without errors.
+
 ## Conclusion
 
 In this article you saw the main steps to generating prompts that lead to relevant AI-powered search results:
 
 - Do not use the default `documentTemplate`
 - Only include relevant data
 - Truncate long fields
+- Add guards for missing fields