bgpd: fix show rpki json void output

bgpd: fix show rpki json void output

"show rpki XX json" should not return a void output because json.loads()
considers it to be an incorrect JSON.

> >>> json.loads("")
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   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)
> >>> json.loads("{}")
> {}

Return "{}" instead in such a case.

Link: https://github.com/FRRouting/frr/pull/15034
Fixes: dff41cc8a9 ("bgpd: Add JSON output for `show rpki prefix` and other show commands")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>