fix: Make the return value of accessToken nullable

Author: dshukertjr

src/SupabaseClient.ts

@@ -51,7 +51,7 @@ export default class SupabaseClient<
   protected storageKey: string
   protected fetch?: Fetch
   protected changedAccessToken?: string
-  protected accessToken?: () => Promise<string>
+  protected accessToken?: () => Promise<string | null>
 
   protected headers: Record<string, string>
 

src/lib/types.ts

@@ -77,7 +77,7 @@ export type SupabaseClientOptions<SchemaName> = {
    * Create another client if you wish to use Supabase Auth and third-party
    * authentications concurrently in the same application.
    */
-  accessToken?: () => Promise<string>
+  accessToken?: () => Promise<string | null>
 }
 
 export type GenericTable = {