[css-variables] Since Syntax now trims whitespace, explicitly make <<declaration-value>> optional so you can still have an empty var. Relates to w3c#774.

Author: tabatkins

css-variables-1/Overview.bs

@@ -58,7 +58,7 @@ Defining Custom Properties: the '--*' family of properties</h2>
 
 	<pre class='propdef'>
 	Name: --*
-	Value: <<declaration-value>>
+	Value: <<declaration-value>>?
 	Initial: (nothing, see prose)
 	Applies to: all elements
 	Inherited: yes