get_by_str_path returns None for root element of a tree

[mwildehahn]

https://discord.com/channels/1069799218614640721/1324562635420733440/1359060718241976390

In [70]: doc.get_deep_value()
Out[70]:
{'test': {'root': [{'parent': None,
    'meta': {},
    'id': '1@1550711757748329106',
    'index': 0,
    'children': [{'parent': '1@1550711757748329106',
      'meta': {'hi': 'world'},
      'id': '2@1550711757748329106',
      'index': 0,
      'children': [],
      'fractional_index': '80'}],
    'fractional_index': '80'}]}}

In [73]: print(doc.get_by_str_path('test/root/0'))
None
In [103]: print(doc.get_by_str_path('test/root/0/0'))
<builtins.ValueOrContainer_Container object at 0x110f82d30>

In [104]: print(doc.get_by_str_path('test/root/0/0').container)
<builtins.LoroMap object at 0x111afabe0>

In [105]: print(doc.get_by_str_path('test/root/0/0').container.get_deep_value())
{'hi': 'world'}