[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/okhttp-gson/.openapi-generator/FILES

@@ -10,6 +10,7 @@ docs/BodyApi.md
 docs/Category.md
 docs/DataQuery.md
 docs/DataQueryAllOf.md
+docs/DefaultValue.md
 docs/PathApi.md
 docs/Pet.md
 docs/Query.md
@@ -51,6 +52,7 @@ src/main/java/org/openapitools/client/model/Bird.java
 src/main/java/org/openapitools/client/model/Category.java
 src/main/java/org/openapitools/client/model/DataQuery.java
 src/main/java/org/openapitools/client/model/DataQueryAllOf.java
+src/main/java/org/openapitools/client/model/DefaultValue.java
 src/main/java/org/openapitools/client/model/Pet.java
 src/main/java/org/openapitools/client/model/Query.java
 src/main/java/org/openapitools/client/model/Tag.java