Do not show toc for empty header, e.g. admonition

Resources/Private/Templates/Page/BlogPost.html

@@ -58,9 +58,11 @@
                 </li>
                 <f:alias map="{contentSections: pageSections.0.content}">
                     <f:for each="{contentSections}" as="contentEntry">
+                        <f:if condition="{contentEntry.data.header}">
                             <li>
                                 <a href="#c{contentEntry.data.uid}">{contentEntry.data.header}</a>
                             </li>
+                        </f:if>
                     </f:for>
                 </f:alias>
                 <f:if condition="{acknowledgements}">