[css-display] Specify that empty text nodes don't generate text runs. Fixes #1808.

Author: tabatkins

css-display-3/Overview.bs

@@ -90,8 +90,10 @@ Introduction</h2>
 	for each property,
 	the value from the box to which that property was applied.
 
-	Similarly, each contiguous sequence of sibling <a>text nodes</a> generates a <a>text run</a>,
+	Similarly, each contiguous sequence of sibling <a>text nodes</a> generates a <a>text run</a>
+	containing their text contents,
 	which is assigned the same styles as the generating <a>text nodes</a>.
+	If the sequence contains no text, however, it does not generate a <a>text run</a>.
 
 	An <dfn for=CSS lt="anonymous|anonymous box" export>anonymous box</dfn> is a box that is not associated with any element.
 	<a>Anonymous boxes</a> are generated in certain circumstances