[BUG] JAVA Spring HashMap is assigned to Object 4.0.0-beta3

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 4.0.0 beta3?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Generation of complex real-world third party API has multiple issues. In 3.3.4 we could work around them but in 4.0.0-beta3 at least one issue is blocking.

This is an example of the generated code that fails to compile.
If you'd like a small test case I can extract it but I thought you might be interested to see the full generation.

  @JsonProperty("_links")
  private LinksAll links = new HashMap<>();

in 3.3.4 this was set to null and so was OK..

I made a skeleton of the application build here.
https://github.com/mjeffrey/psd2

openapi-generator version

4.0.0 beta-3. Regression from 3.3.4

OpenAPI declaration file content or url

https://github.com/mjeffrey/psd2/tree/master/src/main/resources/openapi

Command line used for generation

https://github.com/mjeffrey/psd2/blob/master/pom.xml

Java 8, swagger-core 1.5.22

Steps to reproduce

check out and build the project https://github.com/mjeffrey/psd2 and build with
mvnw clean install

This is the output from the build for each version.
https://github.com/mjeffrey/psd2/blob/master/mvn-4.0.0-beta3.txt
regression from
https://github.com/mjeffrey/psd2/blob/master/mvn-3.3.4.txt

Start by checking out the linked psd2 reproduction and its pom.xml, then run mvnw clean install to confirm the Java generation failure. Compare the generated model and the mvn-4.0.0-beta3.txt and mvn-3.3.4.txt outputs, then trace the generator path producing the _links field; done means the generated Java compiles without regressing the 3.3.4 behavior.