bug: adding empty section

[fominok]

Hi, it seems there is a bug

I do:

org.write_org(std::io::stdout()); // to show the content here in this issue 
let mut headline = org.headlines().next().expect("there is a headline");
headline.set_section_content("", &mut org);

I have:

**** curious
Org validation failed. 1 error(s) found:
ExpectedChildren { at: NodeId { index1: 5, stamp: NodeStamp(0) } } at Section
thread 'main' panicked at /home/yolo/.cargo/registry/src/index.crates.io-6f17d22bba15001f/orgize-0.9.0/src/validate.rs:203:17:
Looks like there's a bug in orgize! Please report it with your org-mode content at https://github.com/PoiScript/orgize/issues.

Same with a string made of one whitespace; works fine if there is actual content