bug: Keys with value "0" are dropped in flatten funcs
0 is a valid number, and should not be dropped. We should only drop null values.
See https://gitlab.cta-observatory.org/cta-computing/dpps/bdms/bdms/-/merge_requests/253/diffs#47c17bb92428a6ebbaca9ce8f910bfcd35cd26d0_91_47
issue