[BUG] rest-assured Oper class generation

Created by: Zomzog

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?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

When we generate the client without the supporting file the Oper class is missing.

openapi-generator version

4.3.1

OpenAPI declaration file content or url

Can be reproduce with petshop.

Command line used for generation

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <executions>
                    <execution>
                        <id>service-test</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
                            <inputSpec>${project.basedir}/src/test/resources/OAS30.yaml</inputSpec>
                            <generatorName>java</generatorName>
                            <library>rest-assured</library>
                            <generateSupportingFiles>false</generateSupportingFiles>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

Steps to reproduce

Related issues/PRs

Suggest a fix

I will try a PR.