It depends as well on your database setup. If your table is set up, and the column has "NOT NULL" set, the empty value (whether it be NULL or an empty string ""), will be inserted into the database as an empty string. If the column is set to NULL, then it will depend on your query.
A query to insert a NULL value:
INSERT INTO table VALUES ( 'value 1', NULL )
So if you are inserting an empty string with quotes around it, I think it will still count as as being a value and just insert the empty string (even the column is a NULL column)
By the way, if you are wondering, NULL columns have a use when it comes to counting. So say the field "value" in my table has some null entries, if I run
SELECT COUNT(value) FROM table
It will only return a count of the non-null entries, null entries are ignored in this case.