[CLI] Fix help command

Created by: ackintosh

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Description of the PR

This PR fixes https://github.com/OpenAPITools/openapi-generator/issues/1278 .

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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)