Number of sections should default to 0

[christoff-1992]

The number of sections should default to 0 to stop subsequent calls been made to other delegate methods when there are no items to be displayed.

https://github.com/zenangst/Blueprints/blob/c3dc5442d3102d933bc439c5c845b277b20034b1/Sources/Shared/Core/BlueprintLayout.swift#L38

This would cause a crash if headers/footers where been dequeued.

[zenangst]

Very interesting find mate!