[cssom] (1) throw exception of attempting to serialize an identifier or string containing U+0000;

(2) improve language of common idiom serialization rules;
(3) replace use of 'unset' with 'clear', define 'set'/'clear' usage in terminology section;
(4) format 'null' as code;
(5) improve language of style sheet state itmes (6.1);
(6) improve prose w.r.t. nullable style sheet state items (6.1);

Author: skynavga