Update dependencies with dangling rubygem_id to null
Update dependencies with dangling rubygem_id to null

Ideally, these should have never existed as we update dependencies
to unresolved_name and reset rubygem_id when a rubygem record gets
deleted.
These seems to have been created before the time we added before_destroy
hook.

Fixes following exception on transitive dependencies page:
```
NoMethodError: undefined method `name' for nil:NilClass
```

sonalkr132/rubygems.org,sonalkr132/rubygems.org,sonalkr132/rubygems.org,rubygems/rubygems.org,sonalkr132/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org