Fix data structure for element_types

[JorgeMrtnzG]

Currently, commit 3585958 removes the checker for empty element_types. This causes that Point, Lines and Polygons are collected in by the process_feature_attribute_completeness lambda function

Users would need to specify within the frontend side which element_type should be collected. For multiple elements, all of them must be specified, then the backend will need to do the validation.