fix(nav-bar): throws exception when indexing null tabs variable
fix(nav-bar): throws exception when indexing null tabs variable

- `MdNavBarController._getTabs()` returns an empty array instead of `null`
  to match existing return type
- rename `MdNavItemController._focused` to `MdNavItemController.isFocused`
  - make it non-private since it is accessed by `MdNavBarController.onFocus()`
- fix/improve types
- remove unused variables
- add JSDoc

Relates to #11964

oostmeijer/material,marekmicek/material,marekmicek/material,marekmicek/material,bluebirdtech/material,bluebirdtech/material,oostmeijer/material,angular/material,marekmicek/material,bluebirdtech/material,oostmeijer/material,angular/material,angular/material,oostmeijer/material,angular/material,bluebirdtech/material