Fix exception in `bootExtensions`
Fix exception in `bootExtensions`

Frontend extenders exist in a weird state of limbo, where they are technically defined, but aren't used or tested at all. In da5db714c2ba49f789bea68971a2b46c29ff6b0a, we shifted from passing `extension.extend` to `flattenDeep` to calling `flat` on `extension.extend`. If an extension doesn't define extenders (as is the case for most extensions), the change breaks the forum. All we do here is add a null check.

flarum/core,flarum/core,flarum/core