I need to show a button that will take the user to the screenshot gallery.
But when there are no screenshots yet should I disable the button or leave it active for the user to click and enter an empty gallery with some appropriate text within?
Leave the button active as an inactive button can cause frustration and lead to exits.
If the user has the intention of visiting the gallery then let them do so, even if it is currently empty.
In the space where the images are going to be displayed, you can show some text simply saying 'Images coming soon' or 'Watch this space!' - whatever type of language best suits your site. Also, using a skeleton UI here could be beneficial to show the user there is in fact going to be content and this is where it will be - a teaser if you will.
You may even want to consider, if the site allows, adding a 'notify me when new images are added' CTA where the user can enter an email address etc a bit like when products are out of stock and you can opt to be notified when come back in stock.