An empty Varchar(Max) column uses a negligible amount of disk space. The script below demonstrates this by creating the table TestTb which contains one column named NullColumn that has a Varchar(Max) data type. When the table is created the column NullColumn is populated with 100,000 rows of Null.

The two readings below show the table when it has just been created and the table with 100,000 rows entered.

IF OBJECT_ID('dbo.TestTb', 'U') IS NOT NULL
	DROP TABLE dbo.TestTb;

CREATE TABLE TestTb (NullColumn VARCHAR(MAX));
GO

sp_spaceused 'TestTb';

DECLARE @i AS INT;

SET @i = 0;

WHILE @i < 100000
BEGIN
	INSERT INTO TestTb (NullColumn)
	VALUES (NULL)

	SET @i = @i + 1
END;
GO

sp_spaceused 'TestTb';

IF OBJECT_ID('dbo.TestTb', 'U') IS NOT NULL
	DROP TABLE dbo.TestTb;