GH-50: Raise error if no topic is given while producing messages

[angad-singh]

Pull Request Template

Description

Changed the publish_list method in lib/deimos/producer.rb to raise error if the topic is blank.

Fixes #50

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Unit Tests
• [x] Using the ItemAttributeChangeProducer in Fadmin

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[dorner]

@angad-singh please fix Rubocop errors and then we're good to merge!

[dorner]

Woot, thanks!