[Eager Execution] Track null values as completed in EvalResultHolders

[jasmith-hs]

With the EvalResultHolders, if super.eval() returns null, we should count that as having an evaluation result. Therefore we need a separate boolean to keep track of if there's an eval result rather than simply tracking if the evalResult != null. A strong example is a custom void function. It will return null when evaluated, but we want to track that it has been called already (in case of a DeferredParsingException) so that it doesn't get called twice.

Instead of writing test classes for every kind of AstNode, I just wrote a simple one for EagerAstIdentifier that makes sure a null context value will be stored as the eval result and that we know that the AstIdentifier has already been evaluated when it finds that.

This was found/is needed to make a test case pass in #632 after its merge with master due to #629