[BUG] Maven plugin incorrectly marks inputSpec as a required field

[jack-handley]

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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using the maven plugin, I can specify an Open API Spec file using the <inputSpec>./filename.yaml</inputSpec> tag, or I can specify an <inputSpecRootDirectory>./directory-of-specs/</inputSpecRootDirectory> for it to merge the files before generating the code. When I choose the latter option, I get an error saying inputSpec is required, despite the generator ignoring the input when the inputSpecRootDirectory is specified:

        if (StringUtils.isNotBlank(inputSpecRootDirectory)) {
            inputSpec = new MergedSpecBuilder(inputSpecRootDirectory, mergedFileName)
                .buildMergedSpec();
            LOGGER.info("Merge input spec would be used - {}", inputSpec);
        }

        File inputSpecFile = new File(inputSpec);

As such, the inputSpec parameter should be optional, and validation should check that one of inputSpec or inputSpecRootDirectory is specified (perhaps with a warning if both are given)

openapi-generator version

Latest (7.0.1)

Maven pom.xml

            <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <!-- RELEASE_VERSION -->
                <version>7.0.1</version>
                <!-- /RELEASE_VERSION -->
                <executions>
                    <execution>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <configuration>
<!--                            <inputSpec>${project.basedir}/src/main/resources/openapi/api.yaml</inputSpec>-->
                            <inputSpecRootDirectory>${project.basedir}/src/main/resources/openapi</inputSpecRootDirectory>
                            <generatorName>java</generatorName>
                            <configOptions>
                                <sourceFolder>src/gen/java/main</sourceFolder>
                            </configOptions>
                            <skipValidateSpec>true</skipValidateSpec>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

https://github.com/fullstack-jack/openapi-generator-maven-plugin-example

Steps to reproduce

1. Clone example repo: https://github.com/fullstack-jack/openapi-generator-maven-plugin-example
2. Run mvn generate-sources to see required field error
3. Edit pom.xml to pass an arbitrary inputSpec e.g. <inputSpec>xxx</inputSpec>
4. Rerun mvn generate-sources to see it create sources correctly despite useless inputSpec

Related issues/PRs

none

Suggest a fix

I might get round to creating a PR some time this week but support from anyone else is very welcome.

[wing328]

cc @borsch - author of #14387