[BUG][Java] Syntax errors in generated code for Atlassian Confluence spec (swagger works)

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

The generator generates Java code like this, which fails to compile:

    if (this.space == null) {
      this.space = ;
    }

Note that using this swagger-codegen with this same spec generates valid code.

openapi-generator version

Tested with 5.3.0 (released) as well as a recent build on master (64a478a93c1163f816634acb81a1e35283ed71f1)

Steps to reproduce

To reproduce,

openapi-generator-cli generate --input-spec https://developer.atlassian.com/cloud/confluence/swagger.v3.json \
                         --generator-name java \
                         --config confluence-cloud-openapi-config.json

And here's the config file:

{
  "lang": "java",
  "modelPackage": "com.atlassian.confluence.cloud.model",
  "apiPackage": "com.atlassian.confluence.cloud.api",
  "invokerPackage": "com.atlassian.confluence.cloud",
  "groupId": "io.soteri",
  "artifactId": "confluence-cloud-api",
  "library": "resttemplate",
  "dateLibrary": "java8",
  "java8": true,
  "hideGenerationTimestamp": true,
  "developerName": "",
  "developerEmail": "",
  "developerOrganization": "",
  "developerOrganizationUrl": "",
  "scmConnection": "",
  "scmDeveloperConnection": "",
  "scmUrl": ""
}

Run openapi-generator-cli with the Atlassian Confluence spec URL, generator-name java, and the shown confluence-cloud-openapi-config.json configuration to reproduce the invalid assignment. Trace the generated model code back to the Java generator templates or entry points involved; done means the generated Java code compiles without an empty assignment.