fix: re-enable tiles healthcheck

[kad-korpem]

Description

Add guard clause for the eventuality that Collection.Tiles struct is nil, refactor newHealthEndpoint() to fallback to basic endpoint when this occurs.

Type of change

(Remove irrelevant options)

• Bugfix
• Refactoring

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR