Add null as posible token value for RefreshResult

At least one of the providers (Google) returns no new refresh token upon refresh. Instead, the previous one can be used again.

Author: henninghall

index.d.ts

@@ -57,7 +57,7 @@ export interface RefreshResult {
   accessTokenExpirationDate: string;
   additionalParameters?: { [name: string]: string };
   idToken: string;
-  refreshToken: string;
+  refreshToken: string | null;
   tokenType: string;
 }