fix cursor movement through empty structural boxes

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/cursor/MoveDown.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/cursor/MoveDown.java
index df90bb0..b134f0f 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/cursor/MoveDown.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/cursor/MoveDown.java
@@ -208,7 +208,7 @@
 
 		final IContentBox childBelow = handleSpecialCaseMovingIntoLastLineOfParagraph(findClosestContentBoxChildBelow(parent, x, y), x, y);
 		if (childBelow == null) {
-			if (containsInlineContent(parent)) {
+			if (containsInlineContent(parent) || parent.isEmpty()) {
 				return findNextContentBoxBelow(parent, x, y);
 			}
 			return parent;