Add description for why the discriminator does not have a nullability-annotation

Author: Mattias-Sehlstedt

modules/openapi-generator/src/main/resources/Java/pojo.mustache

@@ -70,6 +70,9 @@ public class {{classname}} {{#parent}}extends {{{.}}} {{/parent}}{{#vendorExtens
   {{^isDiscriminator}}
   {{>nullable_var_annotations}}{{! prevent indent}}
   {{/isDiscriminator}}
+  {{#isDiscriminator}}
+  // The discriminator does not have Nullability-annotation since it is added during serialization by the @JsonTypeName annotation
+  {{/isDiscriminator}}
   {{#vendorExtensions.x-field-extra-annotation}}
   {{{vendorExtensions.x-field-extra-annotation}}}
   {{/vendorExtensions.x-field-extra-annotation}}