Return null duration instead of default from stopCollectingCacheInfo

[timkelty]

Description

If something calls stopCollectingCacheInfo, there is no way to know if the elements had a duration, or if the default duration from config was used.

This moves the application of the default to \craft\services\TemplateCaches::endTemplateCache.

Use-case

Craft Cloud uses \craft\services\Elements::stopCollectingCacheInfo to collect tags and expiry for static cache. It has it's own default for static cache (1 year), but because a duration is always returned (even if it is just the default config fallback), there isn't a way to know if it actually came from an element.

Notes

To avoid this kind of thing, future versions should separate cache duration settings by use-case, e.g. templateCacheDuration, instead of sharing one config value.

[brandonkelly]

This is a breaking change, so retargetted to 6.x.