NullPointerException with command line interface and command help

Created by: ndjensen

Description

If you run the command line generator with one option of help, it throws a NullPointerException when it can't find the second option. java -jar openapi-generator-cli.jar help Ideally it would return a list of options that help will work with, such as generate. (I have no idea what other options you can provide the help command, hence a list would be nice).

Exception in thread "main" java.lang.NoSuchMethodError: com.google.common.base.Objects.firstNonNull(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
	at io.airlift.airline.GlobalUsageSummary$2.apply(GlobalUsageSummary.java:100)
	at io.airlift.airline.GlobalUsageSummary$2.apply(GlobalUsageSummary.java:97)
	at com.google.common.collect.Iterators$6.transform(Iterators.java:785)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
	at io.airlift.airline.UsagePrinter.appendTable(UsagePrinter.java:57)
	at io.airlift.airline.GlobalUsageSummary.usage(GlobalUsageSummary.java:96)
	at io.airlift.airline.GlobalUsageSummary.usage(GlobalUsageSummary.java:52)
	at io.airlift.airline.Help.help(Help.java:57)
	at io.airlift.airline.Help.help(Help.java:50)
	at io.airlift.airline.Help.run(Help.java:25)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)

openapi-generator version

3.3.1

OpenAPI declaration file content or url

N/A

Command line used for generation

java -jar openapi-generator-cli.jar help

Steps to reproduce

Run java -jar openapi-generator-cli.jar help in a terminal.

Related issues/PRs

Couldn't find one.

Suggest a fix/enhancement

See exception stacktrace. If help is the first argument, verify there is a second argument, otherwise return a list of commands help works with.