[css-animations-1] Make @keyframes name disallow empty <string> (w3c#13151)

* The @keyframs-name style rule should not allow empty <string>.

* Apply suggestion from @tabatkins

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Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: javifernandez

css-animations-1/Overview.bs

@@ -227,7 +227,12 @@ Declaring Keyframes</h2>
 	As normal for <<custom-ident>>s and <<string>>s,
 	the names are fully <a>case-sensitive</a>;
 	two names are equal only if they are codepoint-by-codepoint equal.
-	The <<custom-ident>> additionally excludes the ''animation-name/none'' keyword.
+The <<custom-ident>> additionally excludes the ''animation-name/none'' keyword.
+The <<string>> additionally excludes the empty string 
+(but allows the string "none" and other excluded keywords).
+When serialized, the value is serialized as an <<ident>>
+unless it's a disallowed keyword,
+in which case it's serialized as a <<string>>.
 
 	<div class=example>
 		For example, the following two ''@keyframes'' rules have the same name,