Support 'empty' type

[jim-h-wang]

func NewEquitySwapProduct makes use of EmptyEquitySwapMasterConfirmation2018(), which returns a null or empty object. We need to be able to support passing in a generic empty object into functions.

func NewEquitySwapProduct: <"Function specification to create an Equity Swap according to the 2018 ISDA CDM Equity Confirmation Template, based on a minimum set of inputs which can (optionally) include a Master Confirmation Agreement. The inputs represent the minimum set of inputs required to create an Equity Swap, either based on an existing Master Confirmation Agreement or as a stand-alone Equity Swap">
	inputs: 
		underlier Equity (1..1) <"The underlying Equity asset for the swap.">
		masterConfirmation EquitySwapMasterConfirmation2018 (0..1) <"An (optional) pointer to the Master Confirmation Agreement, if any, that holds further inputs to the Equity Swap">
		
	output:
		product Product (1..1)
		
	post-condition: <"Equity and interest rate payouts must be set-up according to their corresponding payout specifications, and other payout types must be absent.">
		if masterConfirmation exists then
			product -> contractualProduct -> economicTerms -> payout -> equityPayout = NewSingleNameEquityPayout( underlier, masterConfirmation ) and
			product -> contractualProduct -> economicTerms -> payout -> interestRatePayout = NewFloatingPayout( masterConfirmation )
		else
			product -> contractualProduct -> economicTerms -> payout -> equityPayout = NewSingleNameEquityPayout( underlier, EmptyEquitySwapMasterConfirmation2018() );
		product -> contractualProduct -> economicTerms -> payout -> cashflow is absent;
		product -> contractualProduct -> economicTerms -> payout -> creditDefaultPayout is absent;
		product -> contractualProduct -> economicTerms -> payout -> forwardPayout is absent;
		product -> contractualProduct -> economicTerms -> payout -> optionPayout is absent;	
		
	post-condition: <"Non-contractual product types must be absent.">
		product -> foreignExchange is absent;
		product -> index is absent;
		product -> loan is absent;
		product -> security is absent;

The resulting syntax will be easier to read and will be more usable as it avoid the need to create function to produce empty objects.

product -> contractualProduct -> economicTerms -> payout -> equityPayout = NewSingleNameEquityPayout( underlier, empty );