add nullValues attribute in Schema and resolve it in ModelResolver

[infomuscle]

Hello,
I thought there could be better way to set null in request example.
I got hint from @CsvValues of Junit, which can set nullValues easily with the annotation attributes.
I raised an issue #4767 and suggest this new feature in this PR.

commit summarize

• Schema: added nullValues attribute
• AnnotationUtils: added nullValues method
• ModelResolver: added resolveNullValues method and call it in resolveSchemaMembers