Fix - Provides a default value for optional init parameters

Created by: alessiozurru

[SWIFT] - The idea is to manage In a more efficiently way the creation of init methods of model objects providing a default nil value for optional parameters.

This is a trivial change to modelObject.mustache template and could be helpful to create Model objects more simply, in particular when they have many parameters. In addition we could avoid possible compilation error for projects that are switching from swagger-codegen tool which provided an init with empty parameters.

With this change we simply provide the '= nil' default value for all params that are optionals

@jgavris @ehyche @Edubits @jaz-ah @4brunu

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