Who? Someone by the name of Lauri Ranta.
Why? Based on @Sathya's comment that it was Unicode... it's three of COMBINING GRAPHEME JOINER (U+034F). From Wikipedia:
The combining grapheme joiner (CGJ), U+034F ͏ combining grapheme joiner (HTML: ͏) is a Unicode character that has no visible glyph and is "default ignorable" by applications.
I'm not sure whether this can be considered a bug on SE's part or not. I'm pretty sure they don't allow whitespace-only names - I suppose it would be better to consider non-printable and invisible characters whitespace too, which they evidently don't.
Not-so-useful information: Notepad++ actually does display something for this character. I suppose they only do so to keep the spacing correct without implying whitespace.
And it turns out that the grey-dashed-circle in black-dashed-square is actually displayed in several fonts (among them Courier New, Times New Roman and Arial) but not others (notably Calibri and Consolas). Now, at least according to the Wikipedia description, displaying something is technically incorrect. However, it does happen. Certain browser/OS combinations, custom fonts and custom font settings will affect the display of this character in your browser.