Where does a single script go in the Taproot tree of a script path spend?

Where does a single script go in the Taproot tree of a script path spend?

If I only have a single script for the script path does the hash of the script get placed at the top of the Merkle tree (instead of the effective Merkle root) or does it get placed in level 1 of the Merkle tree and get hashed up to the Merkle root with another arbitrary hash?