getFirstChild and getLastChild return ObservableArray instead of TreeNode

[Tobias Engelhardt]

getFirstChild and getLastChild return ObservableArray instead of TreeNode.

Minimal reproduction of the bug/regression with instructions:

getFirstChild and getLastChild return ObservableArray instead of TreeNode.

Expected behavior:

getFirstChild and getLastChild return TreeNode.

Versions of Angular Tree Component, Angular, Node, affected browser(s) and operating system(s):

Bug occurs in versions 11.0.0 to ``11.0.2` of the tree.

Other information:

I would be willing to submit a PR to fix this issue

[ ] Yes (Assistance will be provided if you need help to submit a pull request)
[ ] No