Simplify event view bookkeeping and communications

The following discussion from !729 (merged) should be addressed:

@bwynne started a discussion: (+9 comments)

Passing a null pointer as viewContext actually has a functional purpose: to indicate that there are zero relevant EventViews and thus to deactivate the node entirely. For example, used here:

http://acode-browser.usatlas.bnl.gov/lxr/source/athena/Control/AthViews/AthViews/ViewHelper.h#0142

The "else" part (l1146) of the if (viewContext) that you remove from l1134 was what performed this function. It is a requirement for the ATLAS HLT use-case.

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