Fixing a small bug in the RosTopicState.cs where MessageSender would always be null

[P3TE]

Fixing a small bug in the RosTopicState.cs where MessageSender would always be null

Proposed change(s)

Describe the changes made in this PR.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

Provide any relevant links here.

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Documentation update
• Other (please describe)

Checklist

• Ensured this PR is up-to-date with the dev branch
• Created this PR to target the dev branch
• Followed the style guidelines as described in the Contribution Guidelines
• Added tests that prove my fix is effective or that my feature works
• Updated the Changelog and described changes in the Unreleased section
• Updated the documentation as appropriate

Other comments

[LaurieCheers-unity]

By the way, if this is your only change, I hope that means I didn't break your Publisher component too badly! Did my best to refactor it carefully but I'm sure you have usecases I didn't consider...

[P3TE]

@LaurieCheers-unity There wasn't much trouble when pulling in the refactored changes. A couple of naming changes but other than that everything on my end seems to be working fine!

[P3TE]

@LaurieCheers-unity - ooops, spoke too soon:
#211
#212
Haha, they're not big fixes though.