Adjust rendering property

[kirkkwang]

A proposed change

Currently if conditions are not met, #populate_rendering returns an empty array which makes the rendering appear in the manifest as such. I propose to remove rendering from the manifest if it is empty so it can be cleaner.

• add guard for manifest.rendering
• refactor #populate_rendering
• disable rubocop PerceivedComplexity
• adjust specs

[kirkkwang]

@cjcolvar Apparently, be_false and be_true have changed to be_falsey and be_truthy 🤷

I used be false since it was used elsewhere already, let me know if you want it as be_falsey

Anyway, thanks for the suggestion, tests pass on this now!