By selecting the "Include NULL values" against a column, it will include and match any values that are empty AND a null value.


The difference between a NULL value and Empty value:

A NULL value is when there is no value at all. Empty values are still a value.
A NULL value in a relational database is used when the value in a column is unknown or missing. A NULL is neither an empty string (for character or datetime data types) nor a zero value (for numeric data types).  NULL is a special marker used to indicate that a data value does not exist in the database.