What problem is this PR intended to solve?

Closes #2700

Previously this code

Nokogiri::HTML5.parse("<div></div>").css("")

would raise:

nokogiri/css/parser_extras.rb:86:in `on_error': unexpected '$' after '' (Nokogiri::CSS::SyntaxError)

Now:

nokogiri/lib/nokogiri/css.rb:46:in `xpath_for': empty CSS selector (Nokogiri::CSS::SyntaxError)

which I think we can all agree is clearer. The previous exception came from the bowels of racc and I timed out after digging into it for about 20 minutes.

Have you included adequate test coverage?

Yes!

Does this change affect the behavior of either the C or the Java implementations?

No.