Empty state for sections should consider headers/footers

[jessesquires]

It's possible to have a zero-cell section with only headers and footers. (For both tables and collections)

We should update our definitions of "emptiness" for both models.

Follow-up to #48

Empty means:

• Zero cells (empty array of *CellViewModels)
• nil header view
• nil footer view

Changes needed:

• Update isEmpty on section models
• Handle empty sections accordingly. e.g., return 0 in data source methods when appropriate (mostly applies to single-section models)