The most likely reason for such behavior is that the user does not select all the content (e..g, when using the mouse) and thus some content is left over.
I am attaching a short video with the expected behavior of using ctrl+a and then del or backspace.
@Marcel - it is possible that a proxy or some browser extension is adding that the POST. In any case, if you get this consistently instead of an empty string, you can add a check to see if this is all you get and if so - consider it empty.
@j - this is actually not easy at all, because non-breaking spaces can get created by the user (e..g, when holding the space button). They will not be seen by the user, but they are still part of the content and so the editor must not remove them on its own, because they will often be required as part of the content. Nevertheless, you can use the .Text server property or get_text() methods to get the text equivalent of the content. So, if trimming it (e..g, the JS .trim() or C#'s Trim() methods on that string return an empty string - you'll know it's all whitespace.