[BUG] setting cli arguments causes config file to be ignored

⚠️ Important Notice

Please differentiate the bug

This repository is not responsible for the actual code generation. If you have problems with the code generation, please open the bug at OpenAPITools/openapi-generator.

Please also check if the bug is already known before you open a new bug.

🐛 Bug Report:

Describe the bug

you cant seem to mix a config file with some of the cli options.
In this case I'm trying to set --server-variables=major=2 as the version is determined via script

Steps to Reproduce

Steps to reproduce the behavior:

Config File:

{
  "$schema": "../../node_modules/@openapitools/openapi-generator-cli/config.schema.json",
  "spaces": 2,
  "generator-cli": {
    "version": "5.1.1",
    "generators": {
      "server": {
        "generatorName": "aspnetcore",
        "output": "#{cwd}/../.generated",
        "glob": "src/*.{json,yaml,yml}",
        "modelNameSuffix": "",
        "apiNameSuffix": "Base",
        "additionalProperties": {
          "aspnetCoreVersion": "5.0",
          "classModifier": "abstract",
          "buildTarget": "library",
          "isLibrary": true,
          "packageName": "OpenApi.Generated",
          "operationIsAsync": true,
          "operationResultTask": true,
          "operationModifier": "abstract",
          "swashbuckleVersion": "5.0.0"
        }
      }
    }
  }
}

running openapi-generator-cli generate woks as expected

running openapi-generator-cli generate --server-variables=major=2 errors with:
[error] Required option '-i' is missing

running openapi-generator-cli generate --server-variables=major=2 --config=openapitools.json errors with:

Exception in thread "main" java.lang.NullPointerException: generator name must be specified
        at org.apache.commons.lang3.Validate.notEmpty(Validate.java:395)
        at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:482)
        at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:573)
        at org.openapitools.codegen.cmd.Generate.execute(Generate.java:433)
        at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)

Expected behavior

The cli should use the settings in the config file, with any cli arguments added/overriding the values in the file

Operation System (please complete the following information):

• OS: Ubuntu
• Version 20

Package System (please complete the following information):

• npm 6.14.13