Conditions always true in _get_queryables()

[anesson-cs]

Describe the bug
In _get_queryables(), in the statement if md_mapping is not None and isinstance(md_mapping, list), the first condition is always True because md_mapping is either a tuple or a list and its default value is (None, NOT_MAPPED). A new condition must be found instead.

Just below, the statement if search_param is not None is also always True because search_param is set with get_search_param() which returns a string. It may be replaced by if search_param to check is it not an empty string or removed.