[typescript] strictNullChecks errors for Configuration.apiKeys

[grktsh]

Description

When OpenAPI document has security field of apiKey type, openapi-generator generates service classes which have strictNullChecks errors for Configuration.apiKeys.

openapi-generator version

v3.3.3

OpenAPI declaration file content or url

components:
  securitySchemes:
    api_key:
        type: apiKey
        name: X-API-Key
        in: header
security:
- api_key: []

Suggest a fix/enhancement

I'd like to add this.configuration.apiKeys && to the following condition from typescript-angular/api.service.mustache@master

{{#isKeyInHeader}}
        if (this.configuration.apiKeys["{{keyParamName}}"]) {

Here is the definition of Configuration class from typescript-angular/configuration.mustache@master

export class Configuration {
    apiKeys?: {[ key: string ]: string};
    username?: string;
    password?: string;
    accessToken?: string | (() => string);
    basePath?: string;
    withCredentials?: boolean;

    constructor(configurationParameters: ConfigurationParameters = {}) {
        this.apiKeys = configurationParameters.apiKeys;