Provides a default nil value for optional init parameters

The following change can be helpful to create Swift model objects more simply, in particular when they have many parameters.
In addition, projects switching from swagger-codegen to OpenAPI will not encounter compilation errors and will not need to refactor old code

modules/openapi-generator/src/main/resources/swift5/modelObject.mustache

@@ -31,7 +31,7 @@
 {{/allVars}}
 
 {{#hasVars}}
-    {{#nonPublicApi}}internal{{/nonPublicApi}}{{^nonPublicApi}}public{{/nonPublicApi}} init({{#allVars}}{{name}}: {{{datatypeWithEnum}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}?{{/required}}{{#hasMore}}, {{/hasMore}}{{/allVars}}) {
+    {{#nonPublicApi}}internal{{/nonPublicApi}}{{^nonPublicApi}}public{{/nonPublicApi}} init({{#allVars}}{{name}}: {{{datatypeWithEnum}}}{{#required}}{{#isNullable}}?{{/isNullable}}{{/required}}{{^required}}? = nil{{/required}}{{#hasMore}}, {{/hasMore}}{{/allVars}}) {
         {{#allVars}}
         self.{{name}} = {{name}}
         {{/allVars}}