[Swift] Use default values in modelObject initializers

[allezxandre]

PR #6827 introduced nil values in initialisers for non-required parameters, however this behaviour overrides the default values provided to the struct. For instance, this kind of struct would be generated:

public struct ModelObject: Codable {
    public var nonRequiredAttribute: String? = "defaultValue"

    public init(nonRequiredAttribute: String? = nil) {
        self.nonRequiredAttribute = nonRequiredAttribute
    }
}

which would yield:

let model = ModelObject()
print(model.nonRequiredAttribute) // prints "nil" when "defaultValue" is expected

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[allezxandre]

Tagging the technical committee again as I suspect it didn't work the first time when I submitted as draft:
@jgavris @ehyche @Edubits @jaz-ah @4brunu

(apologies if this is redundant and it actually worked the first time)

[4brunu]

Looks good to me 👍