Don't take vertical space if nextButton is null

[oscardelgado]

Is your feature request related to a problem? Please describe.
I don't want a nextButton. I set it to null. But the vertical space it would take is wasted anyway.

Describe the solution you'd like
If nextButton is null, don't bulid a Container with footerContentHeight. Keep it empty, or build it with height = 0.

Describe alternatives you've considered
See above

Additional context
Add any other context or screenshots about the feature request here.