XmlSchemaComplexType.getBaseSchemaTypeName() returns null

The method getBaseSchemaTypeName() only considers XmlSchemaComplexContentExtension to get a base type name while a complex type definition may be either
a restriction of a complex ·base type definition·,
an ·extension· of a simple or complex ·base type definition·.

Note, that there is a ComplexContentRestrictionTest, which gets a baseTypeName from the content model.