Nopg could create not existing headpages

[samuelet]

If any configured headpage exists, headpage fallbacks to the ":start" page even if it's not exists: https://github.com/samuelet/indexmenu/blob/1ba8d3230a1504fccbc2ffa09020944e3e9fb569/syntax/indexmenu.php#L718

Removing this line fixes it, the drawback is that subtrees without headpages are not rendered (the worst case: the whole tree is not rendered if contains any headpage).