differentiate between a key missing and the value of the key being empty

ZnH5MD needs tests for `atoms.info["value"] = []`, ``atoms.info["value"] = {}` or `atoms.info["value"] = None` as well as for `arrays` and `calc.results` in combination with the `value` key not being present there.

see https://github.com/zincware/ZnH5MD/main/tests/test_empty.py