Add empty and null check for the vefctorQuery.matches[0], output nece…

[xqliu]

…ssary log

Summary

Build a Retrieval Augmented Generation (RAG) AI doc

Screenshots (optional)

If no such null check, it will shows follow error when no match in the Vectorize index was found.

URL: http://localhost:8787/?text=What%27s%20the%20best%20pizza
Screenshot:

Text: TypeError: Cannot read properties of undefined (reading 'id')

Which was caused by the follow line

   const vecId = vectorQuery.matches[0].id

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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