`LexborHTMLParser.css` cannot be used in fragment mode on an empty string

[RealOrangeOne]

>>> LexborHTMLParser("", is_fragment=True).css("test")
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    LexborHTMLParser("", is_fragment=True).css("test")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "selectolax/lexbor.pyx", line 404, in selectolax.lexbor.LexborHTMLParser.css
AttributeError: 'NoneType' object has no attribute 'css'

I believe this is because .root is None. In this case, I suspect the correct behaviour is to yield no results.

[rushter]

We can only disallow such input, since otherwise it would require a check in each API method.
An empty fragment basically means no HTML.
Added an error message.