Fix getter methods for Java model with additionalProperties

    When a model class is configured with additionalProperties, getters for
    non-dynamic properties always return null because values have been set as properties
    of the map instead of the class. As a workaround, this commit ensures that we these
    values are available in getters as well.

    This fixes #4970, fixes #5259, and fixes #5187

Author: stianlik

modules/swagger-codegen/src/main/resources/Java/pojo.mustache

@@ -140,7 +140,12 @@ public class {{classname}} {{#parent}}extends {{{parent}}} {{/parent}}{{#parcela
   {{{vendorExtensions.extraAnnotation}}}
 {{/vendorExtensions.extraAnnotation}}
   public {{{datatypeWithEnum}}} {{#isBoolean}}is{{/isBoolean}}{{getter}}() {
+    {{#parent}}
+    return {{name}} == null ? ({{{datatypeWithEnum}}}) this.get("{{name}}") : {{name}};
+    {{/parent}}
+    {{^parent}}
     return {{name}};
+    {{/parent}}
   }
   {{^isReadOnly}}