Your question was already answered, with no content the element has 0 hight. But here is a bit more info.
The list-styles (bullets etc.) counts as generated content. It is (AFAIK) the generated marker box that gives the li its height even when it has no other content inside it (when list-styles is set to something other than none). The default style given is user agent defined.
An element with ‘display: list-item’ generates a principal block box for the element’s content and, depending on the values of ‘list-style-type’ and ‘list-style-image’, possibly also a marker box as a visual indication that the element is a list item.
The list-item keyword causes the element to generate a ::marker pseudo-element [CSS-PSEUDO-4] with the content specified by its list-style properties (CSS 2.1§12.5 Lists) [CSS2] together with a principal box of the specified type for its own contents.