[BUG] ApiClient class introduced by the java resttemplate generator will not pass a SpotBugs validation

Created by: gmcelhoe

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? The problem is easily seen by inspection of the generated sample code or the mustache files
• Have you validated the input using an OpenAPI validator (example)? N/A
• What's the version of OpenAPI Generator used? 4.0.3
• Have you search for related issues/PRs? Yes
• What's the actual output vs expected output? Described below
• [Optional] Bounty to sponsor the fix (example)

Description

The ApiClient class introduced by the java resttemplate generator will not pass a SpotBugs validation.

At https://github.com/OpenAPITools/openapi-generator/blob/3785165e079de8f36f3baf774897feb1065576ba/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache#L622 a check for returnType == null is performed, after the returnType variable was already dereferenced at https://github.com/OpenAPITools/openapi-generator/blob/3785165e079de8f36f3baf774897feb1065576ba/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache#L614

openapi-generator version

4.0.3

OpenAPI declaration file content or url

N/A

Command line used for generation

Steps to reproduce

Related issues/PRs

Related issue is https://github.com/OpenAPITools/openapi-generator/issues/33 , which could catch the problem during the build.

Suggest a fix

The fix is simply to remove the null check at line 622, as it cannot be reached due to the NPE would would happen on line 614.