Fix CUSTOM_MODEL_CLASS fallback
Fix CUSTOM_MODEL_CLASS fallback

We fell back to a value of `false` which would always strip the code.
This commit changes the fallback to `null` when developing the addon,
which leaves the feature flag a runtime concern.

Also move init code to `included` as `isDevelopingAddon()` is not
reliable in `init`.

Co-authored-by: Robert Jackson <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@rwjblue.com>

hjdivad/ember-m3,hjdivad/ember-m3,hjdivad/ember-m3