[XUnit, Moq, EFCore] Unit Testing Question

public class UserService : BaseService
{
    public UserService(AppNameDbContext db, ILogger<BaseService> logger) : base(db, logger)
    {
    }

    /// <summary>
    /// Get all users async
    /// </summary>
    /// <returns></returns>
    public async Task<IEnumerable<UserDto>> GetAllAsync()
    {
        try
        {
            var users = await _db.Users.ToListAsync();

            if (users is not null)
            {
                // TODO: Create extension method for mapping
                var dto = new List<UserDto>();

                foreach (var user in users)
                {
                    dto.Add(new UserDto
                    {
                        Id = user.Id,
                        Email = user.Email,
                        FirstName = user.FirstName,
                        Surname = user.Surname,
                    });
                }

                return dto;
            }

            throw new NullReferenceException(nameof(users));
        }
        catch (Exception)
        {
            throw;
        }
    }
}

public class UserServiceTest
{
    [Fact]
    public void GetAll_ShouldPass()
    {
        var dbContextOptions = SetupTestDatabase();
        var mockLogger = Mock.Of<ILogger<BaseService>>();

        using (var mockDb = new AppNameDbContext(dbContextOptions))
        {
            var userService = new UserService(mockDb, mockLogger);
            var result = userService.GetAllAsync(); // not calling await here, i know. 1 step at a time
            Assert.Equal(3, result.Result.Count());
        }
    }

    private DbContextOptions<AppNameDbContext> SetupTestDatabase()
    {
        var dbContextOptions = new DbContextOptionsBuilder<AppNameDbContext>()
                                    .UseInMemoryDatabase("UnitTestDatabase")
                                    .Options;

        using (var _db = new AppNameDbContext(dbContextOptions))
        {
            // https://www.mockaroo.com/
            _db.Users.Add(new User { Id = 1, FirstName = "Cornelle", Surname = "Dyett", Email = "cdyett0@ocn.ne.jp" });
            _db.Users.Add(new User { Id = 2, FirstName = "Christie", Surname = "Wickson", Email = "cwickson1@smugmug.com" });
            _db.Users.Add(new User { Id = 3, FirstName = "Leanna", Surname = "Burnes", Email = "lburnes2@liveinternet.ru" });
            _db.SaveChanges();
        }

        return dbContextOptions;
    }
}