null is... null, it cannot be joined upon.
Replace null with something else to make it work:
The NullAsValue statement specifies for which fields the encountered NULLs should be converted to values.
By default, QlikView considers NULL values to be missing or undefined entities. However, certain database contexts imply that NULL values are to be considered as special values rather than simply missing values. The fact that NULL values are normally not allowed to link to other NULL values can be suspended by means of the NullAsValue statement.
The NullAsValue statement operates as a switch and will operate on subsequent loading statements. It can be switched off again by means of the NullAsNull statement.
The NullAsValue can be combined with the variable NullValue that specifies which string to use as NULL value. If the variable NullValue is not used, NullAsValue will replace NULLs with empty strings.
The syntax is:
*fieldlist is a comma separated list of the fields for which NullAsValue should be turned on. Using * as field list indicates all fields. The wildcard characters * and ? are allowed in field names. Quoting of field names may be necessary when wildcards are used.
Set NullValue = 'NULL' ;
Load A,B from x.csv;