JSON expects "null" for empty values

Fix incorrect entry for empty values: undef -> null
Update System::Image::Update to recent git status

* update to most recent developent status on Github
  - deal with status slightly better to avoid stuck on reboots
  - fix handle broken downloads
  - improve documentation
* correct homepage to GitHub repository (instead of Unix::Statgrab's HP ^^)
* add dependency to Archive::Peek::Libarchive to avoid external flash process