Changes

Only fetch an online UUID in online mode

The previous code would get an online UUID even in offline mode that
breaks plugins if the player joins.

Example: You want to store data for player "Test" who never joined. An
online UUID is created and you save it using that UUID.

The player Test joins with an offline UUID but that will not match the
online UUID of the saved data.

Adapted from Spigot commit 25b673fd7e418e21eb445a9e39d51baa0c0ab8b6