[Bug] compose AssertionError on key pointing into None-valued DictConfig.

# 🐛 Bug
## Description
Composing a config throws an `AssertionError` in some circumstances where there is a `DictConfig` whose `_content` is `None`.

## To reproduce
Run the following script to confirm that an `AssertionError` is raised:
```python
from dataclasses import dataclass
from typing import Optional

from hydra import compose, initialize
from hydra.core.config_store import ConfigStore
# from pytest import raises


@dataclass
class Child:
    field: bool | int = True


@dataclass
class Parent:
    child: Optional[Child] = None


cs = ConfigStore.instance()
cs.store(name="schema", node=Parent)
cs.store(group="grp", name="option", node={"child": {}}, package="_global_")

# with raises(AssertionError):
with initialize(version_base=None):
    compose(config_name="schema", overrides=["+grp=option"])
```

<details><summary> Here is the full traceback: </summary>

```text
Traceback (most recent call last):
  File "/home/homestar/dev/mrepo/pysc/repro.py", line 25, in <module>
    compose(config_name="schema", overrides=["+grp=option"])
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/hydra/compose.py", line 34, in compose
    cfg = gh.hydra.compose_config(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/hydra/_internal/hydra.py", line 594, in compose_config
    cfg = self.config_loader.load_configuration(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 142, in load_configuration
    return self._load_configuration_impl(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 263, in _load_configuration_impl
    cfg = self._compose_config_from_defaults_list(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 542, in _compose_config_from_defaults_list
    cfg.merge_with(loaded.config)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 490, in merge_with
    self._format_and_raise(key=None, value=None, cause=e)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/base.py", line 231, in _format_and_raise
    format_and_raise(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 488, in merge_with
    self._merge_with(*others)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 512, in _merge_with
    BaseContainer._map_merge(self, other)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 378, in _map_merge
    expand(dest_node)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 342, in expand
    node._set_value(val)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 647, in _set_value
    raise e
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 644, in _set_value
    self._set_value_impl(value, flags)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 677, in _set_value_impl
    self.__setitem__(k, v)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 314, in __setitem__
    self._format_and_raise(key=key, value=value, cause=e)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/base.py", line 231, in _format_and_raise
    format_and_raise(
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 308, in __setitem__
    self.__set_impl(key=key, value=value)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 318, in __set_impl
    self._set_item_impl(key, value)
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 533, in _set_item_impl
    if self._get_root() is value._get_root():
  File "/home/homestar/miniconda3/envs/pysc/lib/python3.10/site-packages/omegaconf/base.py", line 289, in _get_root
    assert isinstance(self, Container)
AssertionError
```
</details>

## Expected Behavior
No `AssertionError`, the call to `compose` should return a config that looks like `Parent(Child())`.

## System information
- **Hydra Version** :  1.3.0
- **Python version** : 3.10.8

## Additional context
This feels related to https://github.com/omry/omegaconf/issues/1035, though it might not be the exact same code path.

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[Bug] compose AssertionError on key pointing into None-valued DictConfig. #2502

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Uh oh!

[Bug] compose AssertionError on key pointing into None-valued DictConfig. #2502

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