Consider dropping error result from ImmutableTree.Export

[elias-orijtech]

ImmutableTree.Export could be simplified to return no error.

Currently, it has two error cases:

https://github.com/cosmos/iavl/blob/64ffc2749400f762d6279db0fd1ccc2b22e69492/export.go#L41-L47

The first case, tree == nil, can be handled by (1) declaring the case a programmer error and accepting the panic or (2) by returning an *Exporter whose Next returns ErrorExportDone immediately.

The second case, tree.ndb == nil, signifies an in-memory database which doesn't warrant an error. Why not skip the call to tree.ndb.incrVersionReaders and proceed with the export?

CC @odeke-em