[BUG] nullable schema is not allowed to contain `null`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used? 5.0.0
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

Generator crashes if nullable enum schema contains null value.

This is allowed by jsonschema and thus openapi spec.

openapi-generator version

5.0.0 branch, 4th Feb 2020

OpenAPI declaration file content or url

rating:
      type: string
      nullable: true
      enum:
        - null 
        - Gibberish

Command line used for generation

openapi-generator-cli  generate --input-spec openapi.yaml --enable-post-process-file --generator-name elm --config openapi-generator.yml  --output openapi-client

Steps to reproduce

There's an error:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'rating'.Please make sure that your schema is correct!
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:473)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:938)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:423)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
Caused by: java.lang.NullPointerException
        at org.openapitools.codegen.DefaultCodegen.postProcessModelsEnum(DefaultCodegen.java:490)
        at org.openapitools.codegen.languages.ElmClientCodegen.postProcessModels(ElmClientCodegen.java:321)
        at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1214)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:468)
        ... 3 more

Reproduce the nullable enum case with the shown openapi-generator command, then inspect DefaultCodegen.java at postProcessModelsEnum and ElmClientCodegen.java at postProcessModels. Confirm that generation no longer crashes for an enum containing null and that the generated Elm client represents the nullable value correctly.