Place a chest with sandstone in the first slot and issue a repeating command block below it with the following command:

execute if block ~ ~1 ~ chest{Items:[{Slot:0b,id:"minecraft:sandstone"}]} run say SUCCESS

The command will not execute if it contains the NBT. This worked in 18w16a

Note:
The command does execute if testing for block alone, just not with NBT

• Tested with Chests and Hoppers