Javadoc
This method will find the indentation for a node sibling to this node.
It will try to find a sibling node before this node first. If there is
no sibling node before this node, it will try to find a sibling node
after this node. If still not found, we will check if this node is
already indented from its parent. If yes, this node's indentation will
be used. Otherwise, the parent node's indentation plus one indentation
will be used. If this node is null or it's a document node or it's a
first level node (node's parent is a document node) the default empty
string will be returned as the indentation.