tree 369f467c8f5acbf70e72df33d677574b1b052f39
parent 2e42e94bd769b5ffbaf4dff5cb307dfe4c25616f
author Martin Fick <martin.fick@linaro.org> 1658350745 -0600
committer Martin Fick <martin.fick@linaro.org> 1662586748 -0600

Use a TaskTree.Node.Invalid instead of nulls

Previously nulls were used in the TaskTree.NodeList subnodes list to
indicate a special invalid use case, use a new TaskTree.Node.Invalid
class to indicate this. This change helps clarify the explicit invalid
case, but also makes unexpected NPEs during development less likely.
Since it is more obvious now when Nodes are invalid, go ahead and mark
invalid Nodes as cacheable for Branch filtering since they are
immutable.

Change-Id: I1551072ed2824a2d596060259d3da98b92abe04e