That little extra space belongs to post category description.
WordPress allows you to add descriptions to your categories. However, many users don’t notice it because they usually create categories when writing a post which doesn’t let them add a description to that.
Head over to Posts » Categories page. If you are creating a new category, then you can simply enter category name and description here and then click on Add new category button.
If you want to add a description to an existing category, then you need to click on the Edit link below that category.
This will take you to category edit screen where you can add a description to your category.
An example (demo) of blog category page with description content can be found here.
However, if you are willing to remove that extra space and leave your category page as is, then you have to add the following snippet to your additional CSS tab.
min-height: unset !important;