Bug: query of subkey of dict that is in a subset of assets results in TypeError

[aqw]

Like #696, this is easiest to explore in our inventory repo.

Some assets contain the following structure:

connectors:
  mDP: 1
  DP: 2 # 1 x out w/ MST
  HDMI: 1
  USB-A: 4
  USB-B: 1
  3_5: 1
  DVI: 1

However, do not have the connectors key.

When addressing the connectors key directly, all is well:

❱ onyo get --keys connectors
 ──────────── 
  connectors  
 ──────────── 
  <dict>      
  <unset>     
  <dict>      
...
  <dict>      
  <unset>     
 ──────────── 

However, addressing a subkey (valid or invalid) fails:

❱ onyo get --keys connectors.DVI
ERROR: 'connectors' is not a dictionary.

Traceback shows TypeError:

❱ onyo --debug get --keys connectors.DVI
...
ERROR: 'connectors' is not a dictionary.
DEBUG:onyo:Traceback (most recent call last):
  File "/home/aqw/git/inm7/onyo/onyo/lib/utils.py", line 92, in __getitem__
    return effective_dict[parts[-1]]
           ~~~~~~~~~~~~~~^^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/aqw/git/inm7/onyo/onyo/main.py", line 543, in main
    args.run(args)
  File "/home/aqw/git/inm7/onyo/onyo/cli/get.py", line 187, in get
    results = onyo_get(inventory=inventory,
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/aqw/git/inm7/onyo/onyo/lib/commands.py", line 83, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/aqw/git/inm7/onyo/onyo/lib/commands.py", line 537, in onyo_get
    results = [{k: r[k] if k in r and r[k] not in [None, ""] else UNSET_VALUE
                           ^^^^^^
  File "/home/aqw/git/inm7/onyo/onyo/lib/utils.py", line 136, in __contains__
    self.__getitem__(key)
  File "/home/aqw/git/inm7/onyo/onyo/lib/utils.py", line 96, in __getitem__
    raise TypeError(f"'{'.'.join(parts[:-1])}' is not a dictionary.") from e
TypeError: 'connectors' is not a dictionary.