AnsiTreeView: Increase the prefix of the last node if it's a List

[nikosportolos]

Currently, the prefix of the last node is misscalculated when its type is a List.
It needs to be increased so it starts at the beginning of the parent node.

Example

┌─────────────────────┐
│_Map<String, dynamic>│
├─────────────────────┘
├── map
│   ├── id: 123
│   └── username: AnsiX
├── empty_map
│   └── (empty)
├── list
│   ├── This
│   ├── is
│   └── AnsiX
└── empty_list
   └── (empty)