FieldType::isEmptyValue( SPIValue $value ) expects the SPIValue instance as returned by API. As the method signature is generic and default implementation can't hint the proper type for a concrete field type, that should then be checked by reusing exisisting acceptValue() template method checkValueType().

Default implementation also compares the $value to null, which is not needed.

Dedicated tests for the method should be added to the abstract unit test case and implemented for all field types.