[MAVEN PLUGIN] fix javadoc, fix sonarlint issues in module openapi-generator-cli

Created by: karismann

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.

Description of the PR

fix mvn verify -f modules/openapi-generator-cli/pom.xml who failed locally (with jdk1.8.0_171) cause of javadoc warning with maven-javadoc-plugin 3.0.1 but not with 3.1.0 anymore (actually no error in circleci but probably cause of #705 (closed)) fix warning line 2 column 2 - Error: <additionalproperties> is not recognized! in circle-ci log in openapi-generator-maven-plugin module fix some sonarlint issues in module openapi-generator-cli