1st level, fixed parent menu should always show menu tree

In the Drupal 7 version of menu_block, blocks configured with a "fixed parent menu" and a "level 1" starting level should always show links. The thinking is that a fixed parent menu tree is the equivalent of a full menu and "level 1" of a menu will always show links; otherwise, there is no way to click deeper into the navigation.

In the D8 version, the block's links only appear if you are on a page inside the requested tree. If you are on a page outside the tree, the block is empty.