initialize
Initializes the TikiClient with the provided user ID.
This function is used to set the user ID for the TikiClient. The user ID is a unique identifier for the user and is required for the TikiClient to operate correctly. The function checks if the TikiClient is configured and if the user ID is not empty before setting the user ID.
Return
A CompletableDeferred object that completes when the TikiClient is initialized. The CompletableDeferred object does not contain any value and is only used to signal that the initialization process is complete.
Parameters
user ID
The unique identifier for the user.
Throws
if the TikiClient is not configured or the user ID is empty.