Remove extraneous branch for empty strings in interpolated string handler CFG

When an interpolated string handler is created for an empty string, and the constructor of the handler uses an out bool success parameter, we generate an unnecessary branch in the CFG for the code, demonstrated in the InterpolatedStringHandlerConversionFlow_16 test. We could take the approach Aleksey detailed in his review to remove that branch, but we will have to wire a CreateDiscardSymbol method through the semantic model to be able to create an IDiscardOperation for the parameter value. This is presently undone.