[REQ] Avoid warning in typescript client generator

[SimoneDalka]

When I try to generate a typescript client setting only --input-spec and --output parameters, openapi-generator-cli prompts a warning with the following message:

[main] WARN  o.o.codegen.DefaultCodegen - The value (generator's option) must be either boolean or string. Default to `false`.

Describe the solution you'd like

Check if the supportsES6 additional property is present before trying to set it inside processOpts() method of AbstractTypeScriptClientCodegen class:

if (additionalProperties.containsKey(CodegenConstants.SUPPORTS_ES6)) {
    setSupportsES6(convertPropertyToBooleanAndWriteBack(CodegenConstants.SUPPORTS_ES6));
}

or

convertPropertyToBooleanAndWriteBack(CodegenConstants.SUPPORTS_ES6, this::setSupportsES6);

Describe alternatives you've considered

In order to make the warning disappear, I have to explicitly set supportsES6 additional property:

openapi-generator-cli generate -g typescript-angular [...] -p supportsES6=false

Additional context

The warning is produced by convertPropertyToBooleanAndWriteBack method inside setSupportsES6 setter:

• openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractTypeScriptClientCodegen.java

  Line 416 in 12dfe8f

  setSupportsES6(convertPropertyToBooleanAndWriteBack(CodegenConstants.SUPPORTS_ES6));