Consider dropping error result from ImmutableTree.Export

[elias-orijtech]

ImmutableTree.Export could be simplified to return no error.

Currently, it has two error cases:

iavl/export.go

Lines 41 to 47 in 64ffc27

	if tree == nil {
	return nil, fmt.Errorf("tree is nil: %w", ErrNotInitalizedTree)
	}
	// CV Prevent crash on incrVersionReaders if tree.ndb == nil
	if tree.ndb == nil {
	return nil, fmt.Errorf("tree.ndb is nil: %w", ErrNotInitalizedTree)
	}

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