How much storage do blank fields take up?

Hello, my initial structure of my app is I had a lot of Things all with their unique fields, but I need to do searches across the multiple Thing types and show the results in one repeating group. Now I’m starting to think I need to combine all of them into one general Thing with a lot of fields.

Is there any downside to doing this? All I can think of is maybe a Thing with a ton of empty fields could take up some kind of storage? Maybe searches would be slower even though most fields are empty?