Can a reaction be used against a unit moving to an empty battlefield?

No, generally speaking, you cannot react to a unit moving to an empty battlefield.

According to RiftJudge FAQ #1645 and FAQ #6337, a standard move—such as a player moving their unit to an empty battlefield—does not open a chain. Because no chain is opened, there is no window for players to play reactions.

Here is how the timing works for this situation:

1. Standard Move: The unit moves to the empty battlefield.
2. Trigger Check: The game checks for any triggered abilities. A standard move that doesn't trigger any abilities does not open a chain.
3. Result: The showdown is staged immediately, and the game proceeds without priority being passed.

Exceptions:

• If the movement is performed via a spell or an activated ability, a chain is opened, and you may react to that spell or ability.
• If any effect triggers as a result of the move (regardless of how the move was performed), that effect opens a chain, and you can respond to that triggered ability.

If you have a specific card or ability in mind that you are wondering about, feel free to provide the name, and I can check if it creates an exception!