Builds a cql filter from a list of cql sub filters
combine_cql_filters(filter_list)
If all values in filter_list are null, will return NULL, else will return character of combined filters. e.g. "time_start AFTER 2020-00-00T00:00:00Z AND country IN ('Nigeria')"
List of characters and NULL objects that represent individual cql filters e.g. list("time_start AFTER 2020-00-00T00:00:00Z", NULL, "country IN ('Nigeria')")