Clear the root node, not the context model itself
Restrict non-strict validation on branch children only

The top-level call to save will determine whether validation
should be strict or not on it's children. However, the child
nodes should default to the strict validation to prevent considering
empty nodes as been valid or invalid condition nodes from being
ignored.

Also, branches without any children are considered invalid on
save.
Array-based values on conditions nodes are checked to be invalid