but the query returned is empty as shown in the image above.
Here is a link to the github repro with latest changes GitHub - itolimaesther/gatsby-prismic
I did a test with your repository. There are two reasons why the query isn't returning anything:
There are still no published documents on the repository. As soon as you publish the draft with the type 'Article' you'll be able to see it in your API response.
The filtering option that you added to your query isn't correct. The eq operator retrieves documents that match the given data. You can search with fields that have text strings, like a Rich Text, UID or Key Text field. Look at this example: