Return no tiers in GET content tiers API

[farhan787]

Fixed GET content tiers api, now returns no tiers if paid membership is not setup.

• There's a clear use-case for this code change, explained below
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test:all and yarn lint)

This resolves the #15759 issue, where we shouldn't return any tiers in GET api/content/tiers if paid membership is not setup by owner.

@ErisDS I'm new to this repo, I've very little to no context of your business to code terminologies.

1. I've used the boolean key if stripe is connected to check if paid membership is enabled or not, let me know if it's correct or needs changes.
2. I'm looking forward to contribute more to the project and any coding style or architectural docs would be really helpful to know the context and implementation of the features.

[allouis]

Hey @farhan787 - thanks for the PR! We're just having some internal discussions about the best way to fix this - sorry for the delay! It's not been forgotten about