Fix nullref in GetCopyToPublishDirectoryItemsGraphPredictor

[dfederm]

This is happening for sfproj projects because in ServiceFabricCopyFilesToPublishDirectoryGraphPredictor, it passes a null publishDir to GetCopyToPublishDirectoryItemsGraphPredictor:

// A service fabric project publishes its dependencies with its own publish dir. However, we're not providing the publish dir
// since that requires cracking open the service manifest to find the correct subdir. Instead we'll just predict the directory.
foreach (ProjectGraphNode projectReference in projectGraphNode.ProjectReferences)
{
    GetCopyToPublishDirectoryItemsGraphPredictor.PredictInputsAndOutputs(projectReference, publishDir: null, predictionReporter);
}