Provides a default nil value for optional init parameters (#6827)

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}}