scripts/rpc: change default value from None to {}

JSONRPCClient.call recieved optional `params` value,
  but then `params` was used in `json.dumps` function (line #159)
  as a dictionary, so `None` was treated as dictionary
  in some cases, which resulted in TypeError.

This patch fixes above issue by changing
  the default value from `None` to `{}`.

Change-Id: I27ca0ccc2d970c0ff9e8117f02203c03e417adf0
Signed-off-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/462070
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>

scripts/rpc/client.py

@@ -141,7 +141,7 @@ class JSONRPCClient(object):
         self._logger.info("response:\n%s\n", json.dumps(response, indent=2))
         return response
 
-    def call(self, method, params=None):
+    def call(self, method, params={}):
         self._logger.debug("call('%s')" % method)
         req_id = self.send(method, params)
         try: