https://github.com/sqlmapproject/sqlmap/wiki/Usage

Turn off payload casting mechanism

Switch: --no-cast

When retrieving results, sqlmap uses a mechanism where all entries are being casted to string type and replaced with a whitespace character in case of NULL values. That is being made to prevent any erroneous states (e.g. concatenation of NULL values with string values) and to easy the data retrieval process itself. Nevertheless, there are reported cases (e.g. older versions of MySQL DBMS) where this mechanism needed to be turned-off (using this switch) because of problems with data retrieval itself (e.g. None values are returned back).