Fix error if no cache tags was registered

[sfsmfc]

If no cache tags was registered, stopCollectingCacheInfo() returns [null, null]. If we try to assign another tag 'element-api' on it, we get the following error: [Error] Attempt to modify property "tags" on null

This patch solves the problem.

Description

Related issues

[WHITE-developer]

@brandonkelly facing this issue too. Any update on it?

[LinneaHarts]

I'm getting this too on one of my Craft 5 sites but not others.

[LinneaHarts]

This patch works