[Java] better default value handling (#14130)

* add test for array default value

* update null return

* minor fixes

* move default value tests to echo api spec

* add new files

* remove unused files

* fix enum array default, add tests

* better array init

* Update modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

Co-authored-by: Leonard Brünings <lord_damokles@gmx.net>

* Update modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractJavaCodegen.java

Co-authored-by: Leonard Brünings <lord_damokles@gmx.net>

* revert the fix

* improve default value handling

* update native samples, add tests

* update samples

* fix tests

* use conditional test for timezone

* add tests to apache http client echo api

* add option to default container to null

* fix map default value

* minor refactoring

* update samples

* fix javadoc

* fix pom.xml

* add tests in java native echo client

* add java apache client echo tests

* fix test

* fix test

---------

Co-authored-by: Leonard Brünings <lord_damokles@gmx.net>

samples/client/echo_api/java/feign-gson/api/openapi.yaml

@@ -301,6 +301,47 @@ components:
       type: object
       xml:
         name: pet
+    DefaultValue:
+      description: to test the default value of properties
+      properties:
+        array_string_enum_default:
+          default:
+          - success
+          - failure
+          items:
+            enum:
+            - success
+            - failure
+            - unclassified
+            type: string
+          type: array
+        array_string_default:
+          default:
+          - failure
+          - skipped
+          items:
+            type: string
+          type: array
+        array_integer_default:
+          default:
+          - 1
+          - 3
+          items:
+            type: integer
+          type: array
+        array_string:
+          items:
+            type: string
+          type: array
+        array_string_nullable:
+          items:
+            type: string
+          nullable: true
+          type: array
+        string_nullable:
+          nullable: true
+          type: string
+      type: object
     Bird:
       properties:
         size: