The action
attribute of the form
element cannot have an empty value specified, the specifications are clear: http://www.w3.org/TR/html5/forms.html#attr-fs-action. Its value must be a valid URL, at least surrounded by white space characters.
However, it isn't mandatory: you can not define it, the from data will be sent to the same URL of the document.
I don't understand Japanese, so I can't tell you anything about the two articles you mentioned, sorry about that. Maybe one refers to an old draft of the specification.