java.lang.NullPointerException when running config-help

Description

The documentation at https://github.com/swagger-api/swagger-codegen#getting-started seems incorrect, and the CLI command surface for the config-help command seems broken. I was just trying to find out what String to use for the "language" to be able to generate a client API for Node.js

Swagger-codegen version

3.0.20

Swagger declaration file content or url

N/A

Command line used for generation

N/A

Steps to reproduce

At first I followed the README and executed this command:

java -jar swagger-codegen-cli.jar help generate

This produces an error:

swagger-codegen: error: invalid choice: 'help' (choose from 'generate', 'config-help', 'meta', 'langs', 'version')

OK.. so maybe I'll try config-help..

java -jar swagger-codegen-cli.jar config-help

This also results in an error:

Exception in thread "Thread-1" java.lang.NullPointerException
        at java.lang.String.concat(String.java:2027)
        at io.swagger.codegen.v3.CodegenConfigLoader.forName(CodegenConfigLoader.java:31)
        at io.swagger.codegen.v3.cli.cmd.ConfigHelp.run(ConfigHelp.java:17)
        at java.lang.Thread.run(Thread.java:748)

Finally realized there's a langs command, but sadly swagger-codegen doesn't support generating client APIs for node.js :(

(which seems odd btw)

Start by reproducing the documented help generate command and the config-help command with swagger-codegen 3.0.20. Read CodegenConfigLoader.java:31 and ConfigHelp.java:17 alongside the CLI command definitions. Done means the documented command surface is accurate and config-help no longer fails with the reported NullPointerException.