Handle default 0 headline

Also support headline within abstract menu.

Configuration/TypoScript/Setup/ContentElements/Basic.typoscript

@@ -23,11 +23,20 @@ lib.contentElement {
                 }
                 value = hidden
             }
+            15 = TEXT
+            15 {
+                if {
+                    value = 0
+                    equals.field = header_layout
+                }
+                value = 2
+            }
             20 = TEXT
             20 {
                 if {
                     value = 100
                     isLessThan.field = header_layout
+                    isPositive.field = header_layout
                 }
                 stdWrap.dataWrap = {field:header_layout} + 1
                 prioriCalc = intval

Resources/Private/Templates/ContentElements/MenuAbstract.html

@@ -6,7 +6,7 @@
     <f:section name="Header">
         {f:render(partial: 'Header', arguments: {
             uid: data.uid,
-            layout: data.header_layout,
+            layout: headerLayout,
             header: '{data.header} {topic.0.data.title}'
         })}
     </f:section>