mgr/dashboard: prometheus rules internal server error

mgr/dashboard: prometheus rules internal server error

After we increase/decrease the count of the node-exporter, we get a 500
- Internal server error from api/prometheus/rules endpoint. On further
  debugging its caused by the jsonDecodder, because I guess the expected
input for the json.loads() is not a json formatted input. So to fix
that issue I can either do an error handling on the json.loads() or I
can move the json.loads() on the already existing try block. I went for
the second approach here.

Fixes: https://tracker.ceph.com/issues/54356
Signed-off-by: Nizamudeen A <nia@redhat.com>
(cherry picked from commit 672e27cbd2d66f87afa5e9e33537f333eadfb243)