The “blank” project template uses one of the Engine Content maps as a placeholder. Since the content folder for the project is completely empty, and the editor must have a map to load on startup, it uses the “OpenWorld” preset map.
If you go to Edit > Project Settings > Maps & Modes, under Default Maps you can see that OpenWorld is selected as the Editor Startup Map and Game Default Map, and hovering your mouse over the entry will show you the file path of that map within the Engine Content folder (this is hidden by default; to see it, click the ‘settings’ gearwheel at the top right of your content browser and check the ‘Show Engine Content’ option).
To get back to the old default startup settings, create a new map, either via File > New Level or just creating one directly in the content browser. Save the new map, then head to Edit > Project Settings > Maps & Modes > Default Maps and set it as the Editor Startup and Game Default map.