[BUG] Incompatible Content type handling leading to NPE

Version 5.2.1 generated Java class ApiClient which contained method

    public String selectHeaderContentType(String[] contentTypes) {
        if (contentTypes.length == 0 || contentTypes[0].equals("*/*")) {
            **return "application/json";**
        }
        for (String contentType : contentTypes) {
            if (isJsonMime(contentType)) {
                return contentType;
            }
        }
        return contentTypes[0];
    }

Version 5.3.x (0 and 1) generates modified body of the method

    public String selectHeaderContentType(String[] contentTypes) {
        if (contentTypes.length == 0) {
            **return null;**
        }

        if (contentTypes[0].equals("*/*")) {
            return "application/json";
        }

        for (String contentType : contentTypes) {
            if (isJsonMime(contentType)) {
                return contentType;
            }
        }

        return contentTypes[0];
    }

Returning null when argument is empty finally causes NPE in existing code. I guess that we should avoid returning as a rule of clean code.

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?
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Description

NPE thrown in a java client code

openapi-generator version

5.3.x
5.2.x works fine

Suggest a fix

instead of
if (contentTypes.length == 0) {
return null;
}

use
if (contentTypes.length == 0) {
return "application/json";
}

Start with the generated Java ApiClient method selectHeaderContentType shown in the issue and compare its 5.2.x and 5.3.x behavior for an empty contentTypes array. Verify that the generated client no longer returns null in this case and that the existing NPE scenario is prevented.