TypeError when deleting unused Rocknet #2814

[phillxnet]

Fix type error by checking db query _set existence using build-in .exists(). It is proposed that the intention here was .count() > 0, but a simple .exists() serves us here.

Fixes #2814
Fixes #2775

[phillxnet]

Testing this PR for #2814 works as intended.

Testing for the suspected related #2775, using the deletion of an existing network interface as a tangential reproducer: as per the following comment in: #2775 (comment), we again have a fix in-that the network interface is successfully removed and there is now Web-UI errors: but we do have the following log artifact there-after:

[26/Mar/2024 14:56:39] ERROR [storageadmin.views.network:564] the JSON object must be str, bytes or bytearray, not NoneType
Traceback (most recent call last):
  File "/opt/rockstor/src/rockstor/storageadmin/views/network.py", line 560, in delete
    config = json.loads(so.config)
             ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/json/__init__.py", line 339, in loads
    raise TypeError(f'the JSON object must be str, bytes or bytearray, '
TypeError: the JSON object must be str, bytes or bytearray, not NoneType

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[EDIT] This log indicated failure, surfaced by the similarly located - but different Web-UI failure addressed specifically by this PR, has now been detailed in the following dedicated issue:

"Adapt net interface delete to 'rockstor' service null config default" #2819

And relates to managing, system-wide, our emerging null config for all service defaults.