Don't panic (on general section for empty ini)

[sirhcel]

• Calling general_section() and general_section_mut() on new ini or one loaded from an empty file panics as shown by https://github.com/zonyitoo/rust-ini/commit/c2ccbefeb28926912ce2b25780a46b137d34d166
• This panic is neither documented or what I expect here
  • Doesn't an empty ini (file) always contain an empty general section?
  • Which is a corner case but it matters for general_section() and general_section_mut()
• This PR adds an empty general section via Ini::default()
  • So an in instance will always have a general section
  • This general section is empty by default
  • This approach covers all paths for getting an ini instance: new(), default(), load_...(), and so on