add additional `if not mixer, then create` for `updatedComponent` part of `AnimationSystem`

Linking

Problem

depending on creation workflow, we're not guaranteed the mixer will exist by the time it hits updatedComponent (we already handle the attachedComponetn and other cases thankfullY)

Solution

add in the extra if to match the other cases

Breaking Change

nope

Notes

...

Required to Merge

[ ] PASS - all necessary actions must pass (excluding the auto-skipped ones)
[ ] TEST IN HEADSET - main dev-testing-example and any of the other examples still work as expected
[ ] VIDEO - if this pr changes something visually - post a video here of it in headset-MR and/or on desktop (depending on what it affects) for the reviewer to reference.
[ ] TITLE - make sure the pr's title is updated appropriately as it will be used to name the commit on merge
[ ] BREAKING CHANGE
- DOCUMENTATION: This includes any changes to html tags and their components
- make a pr in the documentation repo that updates the manual docs to match the breaking change
- link the pr of the documentation repo here: #pr
- that pr must be approved by @lobau
- SAMPLES/INDEX.HTML: This includes any changes (html tags or otherwise) that must be done to our landing page submodule as an effect of this pr's updates
- make a pr in the mrjs landing page repo that updates the landing page to match the breaking change
- link the pr of the landing page repo here: #pr
- that pr must be approved by @hanbollar

Volumetrics-io / mrjs

add additional `if not mixer, then create` for `updatedComponent` part of `AnimationSystem` #666