[cssom] Serialize U+0000 in string as U+FFFD, NOT escaped as code point

This was missed in 061ede6.

Fixes #1105.

Author: zcorpan

cssom/Overview.bs

@@ -221,8 +221,7 @@ below to each character of the given string, followed by
 '"' (U+0022):
 
 <ul>
- <li>If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD)
- <a>escaped as code point</a>.
+ <li>If the character is NULL (U+0000), then the REPLACEMENT CHARACTER (U+FFFD).
  <li>If the character is in the range [\1-\1f] (U+0001 to U+001F) or is U+007F, the character
  <a>escaped as code point</a>.
  <li>If the character is '"' (U+0022) or "<code>\</code>"