Fixing some issues with threading and NPE

After running Sonar on the master branch, some major analysis opportunities were displayed.

This fixes the use of SimpleDateFormat stored as static fields. SimpleDateFormat is not thread-safe, and may retain data across threads.

While there's no indicator that this has caused any issues (these are mostly used for example code), we should follow these best practices.

This also fixes a handful of NPE and other minor issues such as comparing Boolean.TRUE to strings and no wrapping some closeables in try-with-resources.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.