GKeyFile: allow loading from empty strings

GKeyFile supports empty files and also supports loading from the string
"", but will fail with a critical if you try:

  - explicit length == 0
  - data == NULL

length == 0 should always be valid, and data == NULL should be valid in
the case that length == 0, so add some testcases for those and fix the
code up to allow them.

https://bugzilla.gnome.org/show_bug.cgi?id=668756