empty label means this page can not be displayed as menu item

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #112
| License       | MIT
| Doc PR        | not needed

this was the best solution i found. we only display the menu node if the display option is true *and* there is a label. this way `display` can default to true and the user can unset it to hide the menu, or edit the label and it starts appearing.

the only wtf would be when he adds a label and had set display to false previously. but whatever, that really is an edge case.
good point lukas, added this. for menubundle, i will go with isDisplayable, but here i prefer this name as its not a general decision but only for menu. getDisplay can't be changed because of the interface.