Client refreshHandler is type incompatible with Clerk useAuth (used in documentation)

[marcos-acosta]

In the documentation for using Clerk with Triplit (https://www.triplit.dev/docs/auth/clerk#add-the-clerk-token-to-your-triplit-client), we set the client refreshHandler to Clerk's getToken function. But, getToken() returns a promise which may be null, whereas Triplit requires the promise to always return a string. Since this is in the documentation, I figured something needed updating.