[CLI] Fix help command

[ackintosh]

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Description of the PR

This PR fixes #1278 .

---

Since guava 21, firstNonNull is removed but airline 0.7 still using that. so the following error is caused by run java -jar openapi-generator-cli.jar help.

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)

[ackintosh]

cc @OpenAPITools/generator-core-team

[wing328]

@ackintosh thanks for the quick fix, which works for me:

➜  graphql-generator git:(ackintosh-fix-help-command) ✗ java -jar "./modules/openapi-generator-cli/target/openapi-generator-cli.jar" help
usage: openapi-generator-cli <command> [<args>]

The most commonly used openapi-generator-cli commands are:
    config-help   Config help for chosen lang
    generate      Generate code with the specified generator.
    help          Display help information
    langs         Shows available languages (deprecated. use 'list' instead)
    list          Lists the available generators
    meta          MetaGenerator. Generator for creating a new template set and configuration for Codegen.  The output will be based on the language you specify, and includes default templates to include.
    validate      Validate specification
    version       Show version information

See 'openapi-generator-cli help <command>' for more information on a specific
command.