destroy zombie likes when likable is invalid - Bounty 456

[BUG: ActiveRecord::RecordInvalid: Validation failed: Likable can't be blank - Bounty 456](https://assembly.com/coderwall/bounties/456)
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The code will now handle when the zombie like has an invalid likable :smile_cat: 
Actually there is:
`like.user = User.find_by_tracking_code(like.tracking_code)`
If the user is not found => user_id will be blank and AR will raise `ActiveRecord::RecordInvalid `

with 
`like.user_id = User.find_by_tracking_code(like.tracking_code).id`
we will have `undefined method id for Nil:Class`