fix(auth): Address several auth typing inconsistencies (#993)

Author: MathBunny

src/auth.d.ts

@@ -820,7 +820,7 @@ export namespace admin.auth {
      * When not provided in an `admin.auth.TenantAwareAuth` context, the user is uploaded
      * to the tenant corresponding to that `TenantAwareAuth` instance's tenant ID.
      */
-    tenantId?: string | null;
+    tenantId?: string;
 
     /**
      * The user's multi-factor related properties.

src/auth/auth.ts

@@ -99,14 +99,14 @@ export interface DecodedIdToken {
     sign_in_provider: string;
     sign_in_second_factor?: string;
     second_factor_identifier?: string;
+    tenant?: string;
     [key: string]: any;
   };
   iat: number;
   iss: string;
   phone_number?: string;
   picture?: string;
   sub: string;
-  tenant?: string;
   uid: string;
   [key: string]: any;
 }

src/auth/user-record.ts

@@ -148,9 +148,9 @@ export enum MultiFactorId {
  */
 export abstract class MultiFactorInfo {
   public readonly uid: string;
-  public readonly displayName: string;
+  public readonly displayName?: string;
   public readonly factorId: MultiFactorId;
-  public readonly enrollmentTime: string;
+  public readonly enrollmentTime?: string;
 
   /**
    * Initializes the MultiFactorInfo associated subclass using the server side.