Fix data structure for element_types

[JorgeMartinezG]

Currently, commit 35859586e1a93c29a0cafafa0e04c06ba4fa05d2 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.