[css-mixins-1] Make defaultValue member of FunctionParameter non-nullable (#13604)

This is already optional so there's no need to have it nullable as well

Author: Calme1709

css-mixins-1/Overview.bs

@@ -2285,7 +2285,7 @@ interface CSSFunctionRule : CSSGroupingRule {
 dictionary FunctionParameter {
 	required CSSOMString name;
 	required CSSOMString type;
-	CSSOMString? defaultValue;
+	CSSOMString defaultValue;
 };
 </pre>