Fix an out-of-bounds array access when formatting some empty comment

[mickael-carl]

Also make sure to use default options in tests, to be consistent with what jsonnetfmt actually does.

Quick reproducer of the issue:

$ echo "#
  {}" | jsonnetfmt -
panic: runtime error: index out of range [1] with length 1

[coveralls]

coverage: 55.46% (+1.9%) from 53.571%
when pulling 106c8f0 on mickael-carl:mcarl/fix-oob-formatter
into 5b5bcd5 on google:master.

[johnbartholomew]

Thanks for finding and fixing this!