fixes for empty section header and footer heights

For grouped table view for empty sections we should use CGFLOAT_MIN as sections header/footer height.

More info here:
http://stackoverflow.com/questions/2495936/grouped-uitableview-shows-blank-space-when-section-is-empty
http://stackoverflow.com/questions/19056428/how-to-hide-first-section-header-in-uitableview-grouped-style