Sqobject columns cleanup

[LucaNicosia]

Description

This PR contains some fixes about the columns returned by the sqobject functions:

• empty dataframe columns are now the same as the non empty dataframes (main fix)

• < filters when using build_query_string function

• add columns filtering to assertions
• change some fields in table schema
• avoid always add 'namespace' field if not in schema (tables doesn't have the namespace field)

What's left

• [x] Topology and path tests are failing because I need to craft some ad-hoc tests for them
• [x] I want to reorganize the commits before merging. I left them in this way to drop them easier if necessary

Type of change

• Bug fix (non-breaking change which fixes an issue)