VirtualMapState should not be in VirtualMap if the map is empty

[imalygin]

In the current implementation of Virtual Mega Map there is always an instance of VirtualMapState which contains metadata for the map. There are two issues with this approach:

• a map is never empty and therefore isEmpty method lost its meaning
• it's a manifestation of a "leaking abstraction" - though technically VirtualMapState is an element of the map, it doesn't have the business meaning like other elements in the map and, therefore, should be hidden as much as possible.

A proposed solution is to hide this element when the map is empty

• if it's the only element that is present in the map, isEmpty should return true and size() should return 0