fix: null props in access token

[porcellus]

Summary of change

Fixes handling of nulls in access token payload

Related issues

• access token payload property is omitted when value is 'null' #499

Test Plan

• Added a separate test in session token creation
• Updated other tests that check if access token payload is kept consistent

Documentation changes

N/A, bugfix

Checklist for important updates

• Changelog has been updated
  • If there are any db schema changes, mention those changes clearly
• coreDriverInterfaceSupported.json file has been updated (if needed)
• pluginInterfaceSupported.json file has been updated (if needed)
• Changes to the version if needed
  • In build.gradle
• Had installed and ran the pre-commit hook
• If there are new dependencies that have been added in build.gradle, please make sure to add them in implementationDependencies.json.
• Issue this PR against the latest non released version branch.
  • To know which one it is, run find the latest released tag (git tag) in the format vX.Y.Z, and then find the latest branch (git branch --all) whose X.Y is greater than the latest released tag.
  • If no such branch exists, then create one from the latest released branch.