[R] Handling null values for optional params while building the model json object

Created by: Ramanth

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Description of the PR

Error when optional parameters are not given for the model object

When generating the json of a model using the toJSONString() we encounter an error when we dont pass the optional paramameters (which are of non-primitive type)

Below changes handles the listContainer type optional parameters by replacing them with "null". so that it will be ignored by the endpoint.

Please make necessary changes if required. @wing328 , please look into and take required action.

Example to test: please consider a model which is hanving an example as below

{
	"key1" : "value1",
	"key2" : [  {"k1" : "v1"},
                    {"k2" : "v2"} ],
	"key3" : {  "k3" : "v3",
                     "k4" : "v4"
         	      }
}

In the above example consider key2 as an optional parameter and of type listContainer (i.e, array). Which is throwing an error when we dont provide the value for key2 while generating the model object. Please review this changes and make required changes to handle such scenarios.

Thanks, Ramanth.