Resolve the unclear null apWriter in ReadSingleClusterData

[yunhanw-google]

This is still not making sense to me. If the caller does not care about the value, what do they care about? Just whether it exists?

But looking at the ReadSingleClusterData implemetations:

• The one in TestCommandInteraction.cpp leaves apDataExists unset if apWriter is null.
• The one in TestReportingEngine.cpp, same thing.
• The one in chip_im_initiator.cpp, same thing.
• The one in chip_im_responder.cpp, same thing.
• The one in ember-compatibility-functions.cpp seems to implement the "just set apDataExists" semantics, ok.

And more to the point: is there ever a caller that actually passes null for apWriter? I don't obviously see one.

Originally posted by @bzbarsky-apple in #7634 (comment)

[stale]

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[bzbarsky-apple]

This is fixed now, since ReadSingleClusterData gets a AttributeReportIBs::Builder and hence always has a TLVWriter.