DAOS-16709 test: Handle decoding empty json output (#15397)

Do not raise an exception if parsing empty json output.

Signed-off-by: Phil Henderson <[email protected]>

src/tests/ftest/util/command_utils.py

@@ -714,6 +714,9 @@ def _get_json_result(self, sub_command_list=None, json_err=False,
             self.output_check = prev_output_check
             if json_err:
                 self.exit_status_exception = prev_exit_exception
+        if not self.result.stdout:
+            # Avoid expected json.decode("") exception when there is nothing to decode
+            return {}
         return json.loads(self.result.stdout)
 
     def _get_new(self):