I've noticed that data modify behaves differently since snapshot 19w03a.
In contrast to previous snapshots:

• Trying to write to a non-existent list-NBT-path will no longer generate its path (non-list paths still work).
  Place down any tile entity which has an empty inventory. Run:

  /data modify block <pos> Items[{Slot:0b}] set value {Slot:0b,id:"minecraft:stone",Count:1b}
  

• The attempt to write to a path which already contains the same data will no longer be rejected (You were able to use this to check for NBT equality dynamically).
  Modify the first slot of a tile entity's inventory (assuming it's a stone item):

  /data modify block <pos> Items[{Slot:0b}].id set value "minecraft:stone"