Consolidate decision on empty values in flow-style

Creating this issue in case anyone wants to weigh in on the discussion around support for empty values in flow-style.

References:
 - https://zigforum.org/t/zig-nestedtext-release-0-1-0/383/5
 - https://github.com/KenKundert/nestedtext/issues/23#issuecomment-831195971
 - https://github.com/KenKundert/nestedtext/issues/25#issuecomment-860185422