This patch fixes the crash which happens during execution of `wg-netns list`.
The crash happens if nothing else from `wg-netns` is executed on machine yet,
i.e., after the reboot. In that case, `ip -json netns` returns an empty string,
not `[]`, so we get an exception if we pass empty line into `json.loads()`.
```
$ export WG_VERBOSE=1
$ /home/user/.local/bin/wg-netns list
> ip -json netns
error: Expecting value: line 1 column 1 (char 0) (JSONDecodeError)
Traceback (most recent call last):
File "/home/user/.local/bin/wg-netns", line 391, in <module>
main()
File "/home/user/.local/bin/wg-netns", line 27, in main
cli(sys.argv[1:])
File "/home/user/.local/bin/wg-netns", line 93, in cli
data = json.loads(output)
File "/usr/lib/python3.9/json/__init__.py", line 346, in loads
return _default_decoder.decode(s)
File "/usr/lib/python3.9/json/decoder.py", line 337, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python3.9/json/decoder.py", line 355, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```