Handle lookup returning empty array of users. (#1082)

* Handle lookup returning empty array of users.

* Update tests.

* Use single quotes.

* Update auth-api-request.spec.ts

src/auth/auth-api-request.ts

@@ -629,7 +629,7 @@ export const FIREBASE_AUTH_GET_ACCOUNT_INFO = new ApiSettings('/accounts:lookup'
   })
   // Set response validator.
   .setResponseValidator((response: any) => {
-    if (!response.users) {
+    if (!response.users || !response.users.length) {
       throw new FirebaseAuthError(AuthClientErrorCode.USER_NOT_FOUND);
     }
   });

test/unit/auth/auth-api-request.spec.ts

@@ -369,17 +369,23 @@ describe('FIREBASE_AUTH_GET_ACCOUNT_INFO', () => {
   describe('responseValidator', () => {
     const responseValidator = FIREBASE_AUTH_GET_ACCOUNT_INFO.getResponseValidator();
     it('should succeed with users returned', () => {
-      const validResponse: object = { users: [] };
+      const validResponse: object = { users: [{ localId: 'foo' }] };
       expect(() => {
         return responseValidator(validResponse);
       }).not.to.throw();
     });
-    it('should fail when users is not returned', () => {
+    it('should fail when the response object is empty', () => {
       const invalidResponse = {};
       expect(() => {
         responseValidator(invalidResponse);
       }).to.throw();
     });
+    it('should fail when the response object has an empty list of users', () => {
+      const invalidResponse = { users: [] };
+      expect(() => {
+        responseValidator(invalidResponse);
+      }).to.throw();
+    });
   });
 });