Allow empty-string keys in JSON and YAML input

[johnkerl]

Before

$ echo '{"":""}' | mlr-6.18.1 --j2y cat
[]
mlr JSON reader: object keys must be string-valued

$ echo '- "": ""' | mlr-6.18.1 --y2j cat
[
{}
]

After

$ echo '{"":""}' | mlr --j2y cat
- "": ""

$ echo '- "": ""' | mlr --y2j cat
[
{
  "": ""
}
]