MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
Each function that belongs to this group takes a geometry value as
its argument and returns some quantitative or qualitative property
of the geometry. Some functions restrict their argument type. Such
functions return NULL if the argument is of an
incorrect geometry type. For example, the
ST_Area() polygon function returns
NULL if the object type is neither
Polygon nor MultiPolygon.